Myself, Cara, Maxine Nable, Brandon Qual and countless others use a HI-LATEX tape from China. About 80% of all decent bowlers in Asia swear by it also.
It serves 2 purposes: 1) Ensure that the feel in the thumb hole is exactley the same and is not effected by dirt, sweat, moisture, perpsperation etc. ensuring almost total consistancy.
2) It allows the bowler to exit the ball very quickly which means the ability to drill thumb holes tighter which means a more relaxed swing and free-er motion.
The tape costs about $25 or so for a roll and is available at Brandos pro shop and a few other places around the country. There is a few ways of putting it on depending on personal preferences, most people putting a 1/2inch strip on the top of the thumb from the nail to the base of the thumb (this is the part of the thumb that rubs against the ball on release).
I swear by it and probably would struggle to bowl with out it. It is a special sort of tape though, make sure you get the right one! Similar ones from chemists just dont cut it.