I use white tape both under the fingers and at the side of the thumb hole.
I place 1-2 "staged" pieces of tape under my thumb to form a kind of ramp, so that I do not have to bend the thumb for a good hold. Additionally, the intention is to make the thumb hole tighter, since my thumb holes are drilled with some room, should the thumb swell in summertime.
Additionally, I put 1 piece of tape at the right side of the thumb hole (I am a righty), in about 4 o'clock. This is also meant to make the thumb hole more snug, and I found this position to be the least irritating when I release the ball - in this place, the rough tape does not rub, it just has skin contact when I hold the ball. I'd never put it in the back of the thumb, since it would rub there and also hamper my release.
Tricky. Took some experimenting and trial to work it out for my hand, but the result is very good, much better than the "pure" hole.
--------------------
DizzyFugu - Reporting from Germany
Confused by bowling?
Check out BR.com's vault of wisdom: the unofficial FAQ section
Secrets revealed: What's a fugu?