Trying to hit the overhead shot can be one of the most frustrating experiences in the game. It's a setup ball, waiting to be put away --not. A key to hitting the overhead consistently well is to keep your head up during the stroke.
Resist the temptation to look over into the other side of the court until after you've hit the ball. Your opponent's whereabouts should be of secondary concern to you. If you follow the ball and smack your overhead solidly, you most likely will win the point no matter where he or she is.
Another temptation is to get too excited and swing overly hard in an unnecessary effort to put the ball away with a thunderous hit. When swinging with extra force, you jerk your head down and dump the ball in the net.
Take your time on overheads and, crucially, keep your head up.