If the mousepad is clothbased(ie generic blue lookalike) it's not much you can do, simply because the cloth by time gets saturated by the sweath from you hand. You could try to soak it with some soap and hot water but getting it totally dry again can take some time.
If the mousepad is foam with plastic ontop you can clean it of with isopropanol or some other alcoholbased cleaner. Just rememeber that some of those, like whitesprit, leaves some residue.
As for pure plastic mousemats like
ratpadz the best advice is to first clean them properly useing isopropanol, let it dry properly and then wax it with pledge or some other furniture polish. I do this about every 2 months.
It does not take much time, and you really notice the difference afterwards.
oh, and that ratpadz site got some cleaning instructions too.