Imagine the ball is sitting on top of a tee that is hidden in the sand; your job is to strike the tee and knock it away from under the ball without touching the ball itself.
The two most common reasons for poor bunker shots are quitting on the shot, as mentioned above, and hitting the ball itself instead of the sand behind it. Remember, you do not want to hit the ball at all, the sand will lift the ball out of the bunker as the club swings through it. Stay down and concentrate on accelerating through the sand and finishing your swing.