One of the causes for hitting the ball with the bottom of the club rather than the center of the club is that as you would be swinging into impact what might happen is that the radius of your swing actually breaks down.
Keep Wrists Strong
What I mean by that is that when you are coming into impact either the back of the left wrist will collapse, making a broken line down to the ball, or the left arm might collapse or pull up. Once again causing a broken line between your shoulder, arm and shaft down to the ball rather than a straight line.
Creating an Arc
What you would like to have happen is that when you are coming into impact you would like to have an extension of your left arm and of your left wrist. The left wrist should be in a flat, straight position and the back right wrist should be in a bent position, thereby creating the longest arc you can to reach the bottom of the ball. A real good drill to help you do that is the punch drill.
The Punch Drill
What we are trying to do with the punch drill is make a half a backswing and as we come down we try to get the impression that we are going to punch the ball underneath a tree branch. In doing this what will happen is that you get that extension and you will get that wrist position we need because in order to hit the ball low you are going to have to get your hands in front of the ball.
Keep Hands Forward
This is what the punch drill looks like, hands forward in the swing, half a follow-through. As you can see, I get real good extension of my left arm and wrist and I did not lose the arc that I started with at address. This allowed me to catch the ball solidly in the middle of the club rather than on the bottom.