Try not to get worked up or nervous beforehand, this will probably just make it worse - try and relax!
Also, dont know if it would be possible to either go to the toilet before the interview and rinse your hands in cold water. Or before you go to the interview you could try dusting some talcum powder on you hands - not too much though!
Dont worry about it too much though, alot of peoples hands go sweaty when the have interviews etc. I doubt they will even notice it, and even if they do, if you interview well in shouldnt be a problem!
1. dont worry, ppl will expect u to be nervous and the state of your hands wont affect anything important. 2) the more u worry, the more sweaty your hands will get and 3. slip some cooling moisturiser in your bag
In future, you can get something from the chemist called Dri-Clor which you use for a few days and it prevents it. So you can use it for a while to get you through a situation like that (I used to work in a pharmacy and lots of people asked for it )