Two big keys for my thumber are follow-through, and building up arm strength. Make sure you follow through on each throw, and that will keep,you from hurting your arm/shoulder, and it will ensure that you use your entire body for the throw.
By building up arm strength I like to think of it as if I'm a baseball pitcher. You don't show up in spring training and throw 95 mph, or 9 innings. Start throwing the thumber a little bit in practice and on obvious holes during rounds. After you have done that a while maybe play some thumber and putter only rounds. Try it where you normally wouldn't. You've got to use it to get your muscles and body used to it, and to get where you can hit your maximum distance potential. Make sure you really snap the thumb down on your release. And if you do all that, eventually you'll get to a point where you say, DAMN!, my arm is a little sore or just tired. At that point take a week or two off. No thumbers. Know your body, and let it rest if it hurts or you're tired. But IMO the more you can throw it, the farther and more accurately you will be able to throw it. And if you can get that Schwebby guy to help you, you can just ignore everything I just said. Good luck.
#4 at ZEB is a fun thumber hole. Thumber ace waiting to happen. Nice path from left to right over the little hill.
"What at first was plunder assumed the softer name of revenue"
- Thomas Paine