By "falling off the shot", I assume you mean that you are falling sideways, toward the side where the ball is, as opposed to forward, toward the pins.

If that's the case, and assuming that your timing is pretty much okay (i.e. you are neither terribly early nor terribly late), try moving your sliding foot a little toward the ball as you begin the slide, so that your foot is under the center of your body, and is more able to keep you balanced as you release the ball.  Also, it's easier to maintain your balance if you slide into somewhat of a sitting position, rather than a stand-up-straight position.

Generally speaking, you have to determine why you're falling off the shot.  Once accurately diagnosed, the fix for the problem should be pretty straightforward.
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seadrive
Cogito ergo bowl

You might want to try to put your opposite arm ( left arm for a rightie) out to your side parallel to the ground while in your back swing.  This will help you keep your balance, and help keep your shoulders square to the line so you don't push or pull the shot.  It works for me.  Good luck.
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MMMMMMMM,  Spoiled Eggs.........AaaaarghAaaaargh.......(drooling)

Edited on 6/28/2004 3:01 PM

The problem with what GB2000 said is that if you kick your trailing leg too far inside it will pull you toward the outside and you will have to cross your body to hit your target.

The foot of your trail leg should be just outside of your non bowling shoulder.

I might add that excessive foot speed can lead to pulling up at the end of the shot and make you fall off balance. To fix this try an old Bill Taylor trick called "Belly Walking". Place the ball in the center of your belly and take your stance on the approach. Then just walk toward the foul line in a normal walking speed, then turn and come back to your starting position and bowl and see if the cadence was different.

If you combine a normal walking speed with sliding under the center of your body like Seadrive said, you should be well on your way to sticking the landing.

If you are still having problems you might try shortening the next to last step aka "the power step", and go into your slide toe first, not heel first.