I have an older Squire P-bass and I have great grip on the back of the neck, my thumb does not slip at all. I just recently bought a new Ibanez SRX505 and the back of the neck is slippery, I mean its real smooth...so my thumb slides around a little when I play, it gets annoying. What could I use to get some grip without damaging the bass??
Get some super fine grit sandpaper and roughen up the finish on the back of the neck just a little. It won't show.
Don't grip the bass neck. Just guide your hand with your thumb, you should balance the force of your fretting hand with the body of the bass on your right arm, not with your thumb.