Reply #5: Doing these things will help:

[Spider Jerusalem]

For the weapon's iron sights, you want to be centred on the target...but remember that with the realism factor, your weapon may "pull" a bit to the left or right of dead centre. If you're using a fully automatic weapon (SMG/STG44/BAR), fire in short bursts of about three rounds, as this allows you better control of your weapon (if you don't limit fire to bursts the barrel will drift up and to the right).

Learn to make effective use of cover; know where on a map there are walls, rocks, trees, etc., that you can use to screen yourself.

Don't stay in any one place for too long. Doing so is a good way to have someone sneak up behind you.

If you don't have decent surround speakers, use headphones. It makes a tremendous difference in being able to locate enemies by sound.

Adjust your mouse sensitivity to the highest setting, and spend some time getting used to it. This helps A LOT.

Turn up your screen resolution as high as you can. If you need to lower your texture quality or turn off the eye candy to do it, go ahead. Higher resolution means that you'll be able to spot enemies at a distance more easily (if you're getting popped by headshots from what seems like a ridiculous distance, my guess is that the players doing so have a better graphics card and are probably running at 1600x1200).