Natural Selection/Map-specific Strategies

ns_bast
The Rine start is hard to defend for the Marines, because of its many entrances and cramped space. Atmospheric is a great choice. Other inventive ones are the Tram Tunnel, right in the middle, and hard for a Skulk to rusk in. The Refinery Hive is also a better choice, where it is open.
 * Move out of Rine start

Refinery is the worst hive in NS for aliens. The wide open areas allow jetpacked marines to tear aliens up without retaliation.
 * If attacking Refinery, get jetpacks

Bast has an excellent vent system that can get you directly from one place to another. They should be exploited to your advantage.
 * Know the vents

There is an underwater tunnel underneath the Feedwater Hive that leads to Refinery. It's a fast way to get there. Only aliens can use this route, since marines won't survive swimming through the tunnel. However, aliens will run low on oxygen and require some time to recover lost health.
 * Go underwater

If the marines are remaining in their start location and have welded the vent closed, Gorges can still launch Bile Bombs through a small slit in the vent. An offsense chamber can provide protection against marines that try attacking at close range, however, you should retreat once the marines start launching grenades back into the vent.
 * Bile from the vent

If the marines are pinned down in their start location, you will need to keep control of the elevator. Bring it to the bottom, and build a Sensory Chamber to provide cover for your units. Construct 8-10 Offensive chambers on the elevator, and press the button to raise it to the marine base. The offensive chambers will fire into the base directly, causing massive damage. If they start taking too much damage, you can call the elevator back down to heal them.
 * Elevator attack

ns_caged
The Marine start isn't too bad. It is mostly open, but there are some inventive spots to relocate to. One being behind Processing, in the halls near those weldable grates. You can build your base there a while before being noticed. None of the hives, except for Generator, are really open, but Generator has no nearby res nodes.
 * Relocation - risky, but it could work

The vents in this map provide extremely fast access to different hives. Weld them shut. Especially the one in Shipping and Lower Sewer. Cut them off as soon as possible.
 * Weld vents

Have MCs? Be a Lerk. Get on top of the pipe that's near the main drag from Marine Spawn leading to sewer. Get Silence. You can snipe with Spikes from that pipe a long time before being noticed. Just be sure not to spore - it will give away your position. You may want to drop chambers up there to help you out. You can also put offense chambers up there for an unpleasant surprise.
 * Sniper

Processing is right in the middle of the map, and is a double res node. It would be nice if you could get that. If the other team has it locked down though, leave it be. Its not worth sacrificing yourself and giving the enemies res.
 * Get the double node

ns_eclipse
As soon as the map starts, get an Armory and drop a welder. Get the vent at Horseshoe welded as soon as possible. Lerks can get in that and spore you to death fast, and you're pretty much sunk until you get GLs or JPs - not for a while.
 * Weld the Horseshoe vent

The res nodes in this map are pretty hard to defend. Use this to your advantage. If you're a Marine, Electrify them at first. Also, get a rambo who's a skilled player to go resource tower hunting. This will put you at a huge advantage. As an alien, get the second Hive as soon as possible so you can Bile Bomb those electrified resource towers.
 * Take out resource towers

All 3 Hives in this map are fairly open, especially Computer Core. Jetpack/Shotties are your main weapon of choice here, not HA.
 * Use jetpacks, not HA

There are a lot of opportunities in this map to outrange turrets, or creep out of vents to only be visible to one. A patient Lerk can spike Turrets to death, leaving a blind spot that leaves the TF open to attack.
 * Kill turrets early

ns_hera
Like bast, get out of there quickly. It's not a good place to be. The Holoroom would be an ideal place to relocate to, since it's open and has double res. You want this place badly. Group up and try to relocate to Holo. It's quite hard for the Marines to win without it.
 * Don't stay at your spawn point

The tunnel at the bottom of the elevator near Marine spawn is an awesome place to hide and bite passing Marines, with or without cloaking. It's very, very dark down there, so use that to your advantage.
 * Hide in the shadows