A lot of people use electrical tape, but that might fall into the cheapening category. I make all my own, but I find if you finish them with tung oil, it leaves a slightly tacky, easy to grip surface, plus it brings out the grain of the wood nicely. Of course, if they're already finished, that probably won't help. You could always drill a shallow grid of holes on your chucks and then epoxy in some ball bearings. That would give you 20 or 30 points to grip on each stick depending on how hardcore you got with it, other than that or dremelling grip lines down the whole thing, I don't know.