Not having a completely relaxed thumb throughout the entire swing. Look at Pete Weber at the end of his swing with his hand held high and open. That is what not "thumbing" the ball looks like.
If you can't do this because your thumb hole isn't snug enough, I'd recommend Ron C's Magic Carpet on the back and one or two peices of normal white tape on the front covered with a piece or two of Valentino's Frankentape. It may seem like a lot of work, but once you get the fit right you won't have to adjust it at all because the Magic Carpet is springy and will compensate for normal thumb swelling.
--------------------
ILBT!
( o )( o )