Ha! I'm an expert in this area! My hands sweat more than any other human being on the planet!
They always have, even when i was a little kid. There doesnt seem to be any reason why they do this, but tension can, of course, trigger it.
Sometimes, it will run down my arm and drip off my elbow. The most annoying feature is that if i am not careful, i get a very loud, high-pitched squeak from the nylon strings when i shift.
I played a masterclass for Lily Afshar 30 years ago and she said "Gee, your hands sweat a lot! They really sweat. Wow, your hands sweat a lot." Yeah, thanks, Lily! Like I don't know that already, right?!? I was young and very embarrassed then.
This came up at another masterclass, with Kevin Gallagher this time. He mentioned my string noise and this time i was able to make a joke about it. I demonstrated it for him (and the audience) and told them I was writing a piece that featured the "sweat-squeek" portamento. Kevin thought it was funny and said "Hmm... I can't do that technique!"
Believe me, I have tried all the tricks short of surgery, from powder to chemical antiperspirants to acupuncture. (Acupuncture actually worked, but only for a couple hours at a time. Oh, If i were only rich and famous enough to hire an acupuncturist as part of my entourage!) Breathing and meditative techniques to control tension help too, but I haven't had the energy to really put in some serious work on this.
One BIG thing that helps is to be very prepared for the performance! This should be obvious, but sometimes i forget. If I know the material and am comfortable with my technique, I dont worry as much, hence, less sweating. Usually.
Playing to "Express" myself rather than to "Impress" the audience helps to reduce tension too.
Rubbing my forehead to get "People Grease" on my LH before beginning won't stop the sweat, but it helps the squeaking.
I also sometimes carry a small black towel onstage and unobtrusively wipe my hands on it.
The good thing is that usually once i get started playing, it becomes less of a problem.
I still haven't finished composing my "sweat-squeek portamento" etude, but I will let you know when I do!