Play a lot, and take notes on how players and weapons behave. It actually helps to play different games to understand what you should be looking for.
As an example, the G36E in Combat Arms. You have to learn to feel out the location of the first shot, but you eventually learn that it always fires in the exact same spot, high and to the left of the crosshair. This is an extreme and static example of the kind of things you're looking to pick up on.
In CS:S you want to fire the M4 in a max of 3 round bursts and the AK in 2 rounds. After those initial shots your spread is too large to reliably hit your target. Both weapons spray in a rather reliable arc above the crosshair, which is why you see players aiming low when spraying.
Two exercises:
1. Find an area with small and recognizably delineated targets. A building with windows works really well. Shoot them in order while trying to be as accurate as possible. This trains the hand to move in better coordination with the brain... But it's not the best practice.
2. The 180. Set up a game with bots and stand somewhere facing a wall where you know they'll pop up behind you. When something happens to alert you to their presence, turn around and kill them as quickly as possible. This helps you train that rapid response needed for quick and efficient frags, as well as programming situational awareness.
Keep your crosshair at the correct height for rapid response firing. This is way more important than most people realize. Adjusting to the left or right takes about half as long as adjusting vertically. Similarly, learn how military and police make entry. The concept of pieing a corner will earn you a few kills during every single session; it does for me, at least.
Figure out how players act, and react. All good players are great with a mouse, but all great players are a step ahead of their opponents. When you start noticing these patterns, you'll be flanking the enemy before the call comes out to rotate. When you do rotate, you'll know where they are hiding and how to end them.
If there are multiple targets, focus on the closest first. If you have a bead on a target, you typically do not want to disengage them for another target. The time it takes to adjust your aim creates an inefficiency that results in a lower KD in all but a few circumstances; basically, get the kill you can while you can get it.
My 2 cents for the moment.
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