General strategies
These strategies are useful for any player in Counter-Strike: Source, regardless of their team.
Footsteps
All players run when moving by default in Counter-Strike: Source, and if you're quiet, you can hear their footsteps. This can help pinpoint where people are if they're close to you, or when to shoot if you feel somebody's about to pop out from around a corner. Players who walk instead of running (by holding down Shift by default) make an almost inaudible amount of noise, and so this tactic doesn't work. Crouching will also soften the noise you make.
Flashbangs
Flashbang grenades will blind anyone who sees them go off, or a reflection of them going off; and deafen anyone within a certain radius (to a lesser or higher degree depending on their distance from the grenade). Although they will not hurt anybody, if somebody is caught in a flashbang "explosion", they will be incapable of a co-ordinated response to any attack, and will be an easy kill. Flashbangs should be preferably used against groups of enemies, and you should only attack if you're sure the flashbang has been effective. Flashbangs may be bounced off walls when thrown, but you must always remember that the "hitbox" of any grenade is slightly larger than you might expect, so if throwing one close to a wall (perhaps over a parapet), you must remember to throw it slightly further away from the wall than you might normally do so, to prevent it bouncing back.
If caught in a flashbang "explosion", the best thing to do if you remember your surroundings sufficiently well is to move for cover. If you do not remember your surroundings, you should at least crouch down. If you're completely blinded, it would be useful to reload so you can be prepared for any pending onslaught.
Jumping
Jumping is a very useful tactic. When you're being shot at it's good to jump if you don't mind the accuracy of your shooting falling. If you're reloading, it's especially useful to jump around, as it lessens the chance of you being hit. Jumping around corners (running towards the corner, jumping, and turning in mid-air) is quite a useful trick, as anybody waiting around the corner will be expecting somebody to come in at ground-level, and so any shots fired will hit your less-important chest, as opposed to your head. Jumping also has the added benefit of getting you around the corner faster, so any enemies will have less time in which to react. "Bunny-hopping" is severely ill-advised in Counter-Strike: Source, as jumping slows you down, and repeated jumping will slow you to a crawl.
Weapon accuracy
The accuracy of any weapon is greatly increased if you stand still, and/or crouch when firing, as such a position will allow you to hold the weapon more steadily. With just about any weapon, the accuracy tails off after the first few rounds of automatic fire. Experienced players will usually fire in bursts no more than three rounds long, unless they're at point-blank range, else they lose all the accuracy of their weapon.
Counter-terrorists
These strategies apply only to counter-terrorists, but terrorists might gain useful insight into the way the other team works by reading them.
Bomb site
If you're on the counter-terrorist team, the terrorists plant the bomb, and you know at which bomb site they've planted it (either A or B), make this known to your team by saying either "a" or "b" in team chat. Doing this will help all your team to arrive at the right bomb site, and give you a greater chance of disarming the bomb before it goes off.
Terrorists
These strategies apply only to terrorists, but counter-terrorists might gain useful insight into the way the other team works by reading them.
Planting the bomb
When planting the bomb, a lot can rest on where you plant it. If you know of a good hiding place near the bomb site, make sure to plant it within view of that hiding place. Another strategy is to plant it so it isn't immediately visible to the counter-terrorists; some of them have been known to not notice hidden bombs, and go off to check the other bomb site. One way of doing this is to plant it, then knock over any nearby barrels and roll them on top of the bomb.