This may sound silly but what you probably need is a crosshair or aiming reticle - something for your eyes to focus on. If you have an CRT monitor take a small circular sticker and put it in the middle of the screen so you can focus on that and have a center. If not, use something like game overlay (
http://www.gameoverlay.com) and open a very small window in the background around the center of the screen. Taping two pieces of very fine thread (one centered vertically one horizontally) to your monitor's edges adding some white chalk or paint at the center where they meet can work in a pinch - be creative haha. I remember Mirror's edge did something similar to combat motion sickness (
http://www.joystiq.com/2008/07/17/how-m ... -sickness/) - might work anyways, so give it a shot : p
Have a good one,