Reverse spine tilt
Your upper body is tilted toward the target during your backswing.
From a balance standpoint, a reverse spine tilt forces you to start the downswing with your upper body, which means you can't get enough leverage to maximize your clubhead speed (and that equals lost yards). It also puts a tremendous amount of pressure on your lower spine. See the backward bending just above my belt in the above photo? That hurts!
A line from your chin to your belt buckle should lean away from the target at the top of your swing. Picture a great batter waiting for a pitch. He leans away from the pitcher so when he's ready to swing he can use his legs to create power. If he were leaning toward the pitcher and tried to step into his swing, he'd lose his balance. It's the same with your golf swing: Tilt away at the top so your power is loaded and ready to be unleashed on the way down.
You'll notice an immediate power surge because the elements of your swing will occur in the correct order: Your hips and legs will initiate the motion so you can sling the club through the ball, rather than chop down on it.
The bonus practice tip: Use the sun
Stand with your back to the sun, which will cast your shadow in front of you. Swing to the top, and pay close attention to the shadow of your upper body. Make sure it's leaning away from the target (to your right) at the top of the swing. If it's leaning left, you've got a reverse spine tilt.