Practice.
Know your guns. Learn how each fires. Remember to shoot in bursts. Unlike just about every other FPS, the bullets in CS don't usually end up where the crosshair is.
Practice.
Know your maps. Know where the "hotspots" are. Know where the good ambush spots are.
Practice.
Never go at it from a "Lone Wolf" perspective. Always have some sort of backup. With that said, make sure you leave some space in between yourselves. Nothing sucks worse than trying to move back out of the way of gunfire only to be blocked off by a teammate. This also spreads out enemy fire instead of allowing them concentrate their firepower at one area and wiping your group out.
Practice.
Use the radar screen. Always glance up and make a quick note of where your teammates are. I don't know how many times I've noticed 8 dots in one area and 1 dot in another. As soon as that 1 dot is gone, I know that the enemy is coming in from that area to flank us. Also, use the radar along with sound to determine if someone is close by. With my 4 speaker surround sound, I can tell if an enemy is around the corner or behind me.
Practice.