Haha, I had this problem for a while when I first started performing. My nervousness acted as the perfect grip for cards and anything else. But as performances grew, the more relaxed and natural I felt; hence the nervousness left, leaving me with slippery cards and hands.
Here's what I did before a performance. It may sound very strange, but it worked for me. I would pull out the pockets of my pants and just spritz a little water onto them. Not so much they were soaked, but enough that the fabric would be damp for a long while. When I ditched/loaded something from my pocket, all I had to do was give a little movement with my palm over the damp spot and boom; instant grip.
I know, I'm weird, but that's just the on the spot solution I came up with when I worked professionally.