This condition is really starting to annoy the heck out of me. It causes severe unsteadiness as the whistle just slides along the thumbs at just the wrong moment and screws up the #@!$@$#%$^%$ song that I had almost just figured out that one $@$#@$#@ hard part…
Have you tried one of the thumb rests that are made by Susato? They come in various sizes and are sold at the Whistle Shop at http://www.thewhistleshop.com/
Mike
If the whistle is a cheap or you don’t care that much about the finish, you could use some sandpaper and rough it up a bit. Or spray some silver sparkles on it. If you don’t use a clear coat on afterwards, the sparkles really add some grit.
If it happens most often at gigs or sessions where you are nervous, try spraying your hands with anti-persperant before going out. It will probably cut down on the sweat.
Talcolm powder, either store bought or the kind they use in gyms to lift weights. A bit of that on your hands before you play ought to sort it out!
I have some gorilla snot. I got it here http://www.gorillasnot.com/
It is pretty good stuff and smells like pine resin. And it doesn’t cost much.
It really works too. I got it for when I used to have trouble covering flute
holes.
Hand cream works too if you get some real thick cream but if you use too
much it just makes your hands slippery not tacky. Also someone told me
to rub your fingers on orange peel to get tacky fingers.
A bit of fabric bandaid stuck on the back under the thumb works well as it is rough textured and non-permanent. Elastoplast is what we have here. I often use it for beginner students for thumb placement as well as grip.
Some trad players just lick their thumb (or fingers) for a bit more adhesion - better than sticking them up your nose…