I am a pretty good cs player. I have been playing since 1.5, on and off. Here is a few tips from me:

Learn the maps, its important to know when you will meet the enemy, that way you will be on your toes and not suprised.

I know someone in this thread said aim low, but I aim for the head, and so does most top cs players, I used to watch CAL matches years ago, and they all aimed for the head, its better if you shoot 1 or 2 shots and get a HS then having to shoot more to get it. Plus most guns are amazingly accurate on the first shot.

I know this might be irrelevant but I used to play alot of aim_ maps years ago, to improve aim with the most used guns at long range. I'd suggest scoutzknives to improve sniping skills, fun map and you will learn how to snipe.

I play with a high enough sensitivity that I can turn around real fast if need be, but not much faster. I think too fast makes aiming harder in stressful situations while too slow makes it impossible to shoot the guy behind you.

Turn off automatic pick up for stronger guns option. Got me killed too many times.

I wear headphones and turn the sound up, it is important to know where your enemy is at just by the sound. You should be able to tell where they are and if they are friendly or not within a split second.

I have more but these are probably the more important ones. Good luck at getting better