I use a small permanent marker dot dead center on my CRT. Started doing it back when I was playing Stalker, and they didn't have a gun sight unless you were zoomed in.
Not that I actually use it much in flying, but for GV's it can be quite handy at times.
Another one, if in a tank firing from a slope, I fire a ranging shot, put mouse on the spot where it hits and freeze it. This gives me the angle I need to predict where they will hit.
Gaming the game, by all means yes. Cheating, no not even close.