You can practice a good release by throwing a football underhanded in a tight spiral. Progress to doing the same thing, but only holding the ball with your fingertips.
The swing - free armswing, accuracy (as Oskuposer mentioned), and follow through - can be practiced with something as simple as a softball or a horseshoe (Walter Ray Williams is a champion horseshoe competitor).
Footwork, timing, and position can be practiced in dry socks on a waxed tile floor.
Strengthening the legs is always beneficial to bowling, you really can't go wrong there. Be careful not to build too much upper-body bulk as it can impede the free armswing.
The other crucial elements of a good fundamental game do not require any special devices. For practice in keeping my head still during a free-armed motion, I find golf, darts, and billiards to be excellent avenues of practice. Juggling is wonderful for hand-eye coordination (so important to hitting a target) and dancing or rock-climbing does wonders for balance which is crucial at the line in bowling.
Good luck and thank you.
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J.J. "Waterola Kid" Anderson, the bLowling King : Kill the back row
Edited on 6/2/2005 5:01 PM