Terrorist
You have two options: to stay near the hostages, or to rush to meet the counter-terrorists. Staying near the hostages is safe, but you are open to attack from several corridors, and counter-terrorists will probably appear from more than one at one time or another. This strategy allows you to "protect" the hostages from the counter-terrorists, but could be called camping by more inflammatory players. Rushing to meet the counter-terrorists is risky, as they have many good sniping points around the garage area. If you choose to go down the steps towards the garage, jumping around the corners always helps surprise the counter-terrorists. Grenades and flashbangs bounced around corners are also quite effective, especially if you know somebody is coming up the stairs. You must always be wary of counter-terrorists outside, as the windows smash easily, and there is plentiful cover outside. It would be adviseable to have at least two terrorists rush out through the window first. Opening the garage door from the outside gives a warning to any counter-terrorists inside, who will be waiting for whoever has opened the door: a better bet is to go in through the porch half-way up the stairs, and jump down the last flight into the garage, perhaps letting loose a grenade or flashbang first. Rushing down the main corridor of the office building is almost suicidal unless you're exceptionally fast, especially if the counter-terrorist team has snipers, because the storage rooms opposite the main office building offer superb sniping galleries, and counter-terrorists like throwing grenades in through the front window. A better bet is to go down the corridor at the back of the terrorists' spawn and exit into the small courtyard with the snowman, quietly walking through the passage into the main courtyard and crouching down at the exit porch to snipe any rambling counter-terrorists. Taking this option, you must always be wary of snipers in the sniping gallery. Once your sniping is finished, you can climb up the ladder to the right and follow the counter-terrorists through their likely entry path, shooting them rather unsportsmanshiply in the back before they get to the hostages.
Counter-terrorist
The counter-terrorists have the fun job of assaulting the office building. There are three main routes of assault: the garage, the left-hand side, and the main entrance. Rushing the garage is a safe bet, since there is plentiful cover. It's your choice whether you go outside through the garage door, or rush up the stairs, but if you do go outside you must remember to watch for terrorists through the window. Jumping on top of the crates allows you to shoot terrorists through the window without revealing too much of yourself to enemy fire. Once inside, the route to the hostages is clear, although you might want to take a detour round the inside of the office to confuse terrorists. The main entrance is another easy ride, with cover provided by the curving staircase. A flashbang into the main corridor is helpful unless you know it's clear. The best path to take once inside is to go straight forward, and turn right, taking the back corridor into the hostage area, jumping around all the corners. The left-hand side is risky, with terrorists having the advantage of not having to climb up a ladder before being able to shoot. A good plan is to go round the corridor into the small courtyard with the snowman, and check the room is clear before smashing the window and entering. Once inside, just follow the back corridor, jumping around every corner and throwing a flashbang into the hostage area before entering. When you've got the hostages, exit down towards the garage. When leading hostages to safety, make sure to check for terrorists camping the rescue zone, as it is a common last-ditch strategy, especially by bots. Common hiding places are behind and on top of the skips. A flashbang into the garage wouldn't go amiss.