Some fundamental mechanics to raids, and some common mistakes that have been seen in guilds that don't have a long history of raiding.
Common wipes: Generally, there's only a few different ways raids in EQ totally collapse, and it's important to understand the different underlying reasons for each way, so you know how to best fix a problem.
A. Gutting, or Collapse of a Raid's Support structure. Generally, this is the way a raid wipes on a pull too big for it to handle. If you ask an inexperienced raider, 'What's a raid's defensive classes' they'll usually answer 'The tanks'. This is a bad way to think about raids, or at least not entirely correct. You want to think one level lower than that and say, 'The healers and crowd control'. Say you have a large, 8 pull come in unexpectedly. Here's how a gutting happens: the 2 enchanters on the raids try to mez. They are resisted, mobs immediately aggro on them. The healers try to heal, 4 untanked mobs immediately go on them. Healers and CC are overwhelmed since the entire pull (save the single one on the assist) is basically focused on them. They all die. At this point, usually, every melee is still standing... including most tanks. Then the tanks get aggro. No one is alive to heal them, or if they are, they rapidly get aggro from uncontrolled mobs and die soon after. Note, in this, the tanks die -last-. That's very important to understand. Tanks were the safest characters, and the least in need of support.
There's several ways to approach a Gutting situation like that. The first, and most obvious, is to try to split and do smaller pulls. In fact, a lot of high end guilds today use VERY exploitive pulling or delaying tactics to ensure just that, even in encounters specifically designed for you to have to engage a large amount of mobs at once. Personally, I hate that kind of thing, and tend to try the straight up method- for actual control, there's a few different things to checklist and remember:
1. First, what's mezzable and how resistant are they? There's a point where enchanters trying to mez something just becomes inefficient, because they are too resistant to be reliable or practical. For example, in the Valdoon raid (a death limit raid, I should note), there's some extremely magic resistant shadows. I put bards -only- on mezzing them. Bards generate nearly no aggro with songs, and have a -MR on mezzes. In effect, they can attempt to mez on incoming and be fine. Alternately, in other situations, I'll have shamans paired with enchanters, to assist on the same targets, and only a mezz attempt after malo is in. Lastly, I sometimes have knights, specifically paladins/SKs, use damageless aggro spells to aggro mobs on inc, and hold them until mez is in. Of course, sometimes mez is flat out a bad idea to even try, which lends itself to tanking.
2. Offtank situations. The difficulty here is twofold. One, you need to be positive all tanks have very rapid targeting on incoming mobs (pretargetting is a huge bonus if you can do it), or the untanked mobs will annihilate healers. Two, you need to make sure that tanks can generate enough aggro before they need the first heal, or again, healers will die. Divine Arbitration is extremely powerful for that. Having a near complete heal that's completely aggroless as a starting heal is huge. Similarly, divine intervention is, with high charisma on the target, an EXTREMELY powerful spell. In fact, in a dangerous clearings situation, I usually cycle DI on a tank, a high aggro debuffer, and myself in that order, and keep the 3 up at all times. I make sure all my clerics DI their tank, even people as far as 5 or 6 down a tank order, and themselves as well. Emerald cost does not justify the loss of hours of time from a wipe.
3. Debuffers, this is a tough one. The biggest problem is 'How important is it that debuff is on quickly, and how important is it the debuffer lives'. It can be either or. For example, when I first did Sol Ro, our equipment was bad enough where our tanks would have a devil of a time both holding aggro and living in the few seconds the mob goes unslowed. Now, he could just charge using fortitude before defensive (which is our usual method in hard boss fights), but back then that wasn't an option. So what I did was just have shaman/enchanter pairs suicide slow before we went. Duration was long enough where they could just go, slow, die, drag them, rebuff them, and go well before slow would fade. I did this for an awful lot of difficult encounters (Mithaniel Marr being a standout pain in the ass). In mass fights though, frequently debuffers need to stay up the entire fight. If you have to, tell your debuffers to go -slower- in casting. If the difference between getting slow on in 5 seconds vs 15 seconds isn't significant, there is no reason for them to take aggro and die to some random diaku. Hold them back.
B. Outer collapse or, 'Whoops, the tanks are dead'. This is a tough one to fix because it's always possible you do everything right, and are trying something so far ahead of your gear that you basically have to hope that you don't get a spike and lose. I've had a lot of those. Flawlessly performed, badroll, it's over (see my comment about DI above for hedging those bets). Still, there is some things you can do about it.
1. Let's face it, we all know some clerics that have a zen-like awareness of who is going to take damage, predict it 7 seconds in advance and land a ch 2 seconds into a tank having massive aggro and still manage to avoid getting aggro themselves. We also know clerics that for a minute and a half as you are dying to a low green you say, 'TURN OFF THE TV AND HEAL ME' and then they come back and complain you just 'instantly died'. Not everyone is cut out to be a healer, and some are worthless enough where you give serious thought to suggesting that maybe a melee is more their speed. Sometimes you can fix it (apathy), sometimes you can't (stupidity). But usually, when you can't, as a leader it's just your responsibility to do the best with what you have. I pretty much always build every group on every raid my guild does. That gives me a LOT of control over what healers go where, and to build groups that have double, triple, even quadruple coverage if they need it, while making sure my best clerics aren't covering groups that simply do not need it. Play to your people's strengths. Does the wizard group need a cleric? Then give them a druid and double up 2 strong clerics with the maintank on a tough clearing. Are there a lot of AE's? Then why aren't the paladins spread around, and why aren't the bards covering a rogue group that can benefit the most from the dps jumps while still being protected by Veeshan. Know your members, know their capabilities, and put them where they do the most good. Don't tolerate whining. Your first obligation is to succeed for everyone who isn't complaining, not to group Cleric that afks to cyber Wiz in same group when it obviously does not fit the raid.
2. Similarly, do not allow entitlements to interfere with winning. Sure, everyone likes to be the tank that saves the day, and some people want to have an important job. But if someone will get you killed for making them an offtank, or if that cleric is lagging and simply cannot lead the complete heal chain, do not do it. If I can think of one thing that wipes up and coming guilds the most, it is leaders that are afraid of offending members putting people they KNOW are poorly suited for a role specifically in that role. And listen, you don't have to be an @#%^ about this. You can very calmly, very rationally say that you need X in there for Y and Z reasons. You don't have to say they are a bad player or anything like that. But you still do not have to sacrifice the success of your raid in order to make a few people feel more needed. And you should never, ever tolerate stupidity from guests because you are afraid of offending them.
3. About complete heal chains and keeping a main tank up, there's only so much variation you can do with them really. I personally hate pauses and never use them, because I feel they are dangerous. Other people swear by them. I measure complete heals in exact seconds between the CHs landing, other people measure them by the count between clerics casting (an important but confusing difference- 6 clerics by my method would be a 2 second rotation, since at 10 sec + 2 sec recycle they could cast quickly enough after one another to do exactly that. Other guilds would say a '6 cleric 0 second rotation' to describe the exact same thing. I like my method because it describes the exact rate of healing, while saying '0 sec rot' only is descriptive if you know exactly how many clerics are involved. Sorry for the aside). There's a few things you can do to tune it. Firstly, do you tank swap? In some harder fights you might have to. Carefully organize who is tanking and when defensive is wearing off, second tank ae taunts to keep a smooth tanking transition with nonstop defensive. Some people might think, 'What's so different than the tank just grabbing aggro when the main tank dies?' Well, the difference is the second tank has the rotation slip seemlessly on him by knowing exactly when he is getting aggro and starting the first CH 8 seconds before he does. The second method has him tanking 10 seconds on random heals and praying to god. Often, God tells him to go screw himself and the raid wipes.
4. If you can, chain bulwark of vie and keep Divine intervention on the main tank during a boss. Also bards with AC songs and DSes are the win. Enough said there really.
5. Tanking without a complete heal chain is only done well if you have really, really sharp clerics that communicate with one another very well, and predict things very well. Because healing based on health bar is totally meaningless on any difficult fight, since it won't update until well after the tank is dead. So you need to simulately a complete heal chain by clerics knowing exactly about when the next ch needs to land based on who said what when, and it's still only a real marginal gain in efficiency. The only times you should really not use a CH chain are when the raid is either too easy to make it necessary, or the situation is too chaotic make it effective. Clerics being too proud to do one is stupid, inefficient, and destructive. Clerics doing one poorly or not being able to catch other people's mistakes in one is likewise inexcusable, and will wipe the raid.
C. Lastly, the attritional wipe. This is bad from a morale perspective because usually it looks like your raid was crushed and there wasn't a whole lot you could do about it. Appearances are deceiving. I had a raid against Kelerdrix with 50 people that was slaughtered when all the clerics slowly went oom, and we wiped at an embarrassing 70% or so. We then killed the next day with 40, with less healers, with no deaths and most clerics having 70%+ mana at the win. Small improvements in efficiency can multiply the survivability of the raid.
1. First, groupping. There is no raid where groupping is more important than an AE fight. I strongly, strongly encourage anyone in a 'just grab 5 other people and go' style raiding guild to try focused group building on our next ae-focused encounter. The difference it makes can be overwhelming if your pick up style groups aren't very efficient. Placement of every class is important- are the SKs where they can use their group ATK buffs the most effectively? How about their group mana tap? Who are the bards amplifying and why- do you gain more dps from their atk/overhaste or from muse and why, who needs veeshan the most? Where are the paladins, and who needs their group heals the most? And so on. I can't overstress how big a help this is. People that know their roles need to have an opportunity to do just that.
2. Secondly, resists and preparation. If you are fighting something with chromatic AE, people bragging about their 4 500's and their 1 300 resist are going to get annihilated by the chromatic AE with a significant resist adj. People have to understand what is important for a fight, and adjust accordingly. Some fights I have and would use shaman potions for. Some I need to assign bards with ae songs to cover multiple areas of the fights, to be sure both melee and casters get AE resists. Sometimes you need to use circle of summer's stacking, or DMF. Don't overlook everything you have at your disposal and take damage you can't afford.
3. Mana drain fights- there's a lot of them. First, is there anything mindwrackable in the encounter? A complete heal rotation with a necro for mindwrack is a pretty stable thing, even with some mana drains. Second, if it's cureable, who is assigned to cures? Never, ever trust people to manage this on their own. Third, if you are trusting people to cure themselves, for the love of god call the ae and get the point across they need to cure themselves now, or some dim bulb will be saying, 'oom oom' 30 seconds later. Trust me on that one. Lastly, don't use dispel as a tactic lightly, but there are some encounters where it's a real good idea. Careful about ruling things out.
4. Masses. Use them, get it organized, get everyone with the AA for it. Make sure everyone has the buff slots free to support it in big fights. My clerics seldom use mass anymore for anything BUT mass CR.
D. Lastly, stupidity. Part of the job of a raid leader is to predict stupid things happening and just to warn members not to do it before they do. You'd really, really be surprised how big of a problem this is even for the highest guilds. Generally, I find humor is the best cure for stupidity. Things like, 'Please don't run into the huge dragon ahead of us' works pretty well, and everyone laughs, but I swear to god the one time you don't say it some idiot flips on numlock while grabbing a coke and wipes you.