Getting headshots is actually pretty easy. The most important thing for everyone to understand is why you aren't getting them in the first place. Here are quick tips which will increase your headshots:
1. STOP moving. If you want to hit someone accurately, STOP moving and take a shot (after your crosshairs go back to normal). At a complete stop, after your crosshairs are normal, your gun is deadly accurate.
2. Aim for the chest. Far too many CS players I have played against, watched, etc that are basically looking at the ground. The more you are looking at their chest/neck/head, the better chance you have of hitting those spots.
3. Learn the amount of recoil necessary to hit someone in the head at full auto, at the distance you are firing.
Running headshots happen, but not nearly as often as controlled, burst blasts near the dome. #2 is what I see much more frequently than anything else. New players (and some that have played a long time) tend to look at people's waist and not at the chest. Once you are looking at chest level, just practice looking at chest level through all walls/textures/doors, etc. When you get this down, your quick bursts will drop people FAST. If you can master that, the headshots are mucho easier.
#3 is tough. Up close, aim for the head, no doubt -- the first shot MUST count. When you are at say 10-30feet, that is about medium distance and headshots greatly depend on where you start to fire at them and how much recoil you compensate for. At anything longer than medium distance, single shots are the only real key to hitting someone in the dome on a consistant basis. As much as people may not believe in CS, aiming your shots are the head is very easy to do, even when you are being fired at. Pistols round is where headshots shine, anyone that gets lots of headshots in the first round usually knows what they are doing and will continue to do so throughout the game.
Some people claim some guns are better at making headshots aren't completely wrong, but they do have a point. With #2, many people aim at the incorrect area and their gun of choice's recoil will help them with the headshot. Watch any good player and they'll dish the dome love with virtually ANY weapon.
vash