The backing that comes on the Nexus S is a bit slippery, and certainly prone to nasty fingerprints. An industrious user at XDA has transformed his Nexus S backing from slippery to secure with PlastiDip spray paint. He outlined how he did it:
I simply applied four thin layers of spray on black PlastiDip. As in the instructions, make sure to shake the can thoroughly for a full minute and keep the can of PlastiDip a full 12-16 inches from the back cover of the phone. Wait a full 30 minutes between coats. Make sure to spray all the way around the edge of the battery door into the back of the cover. If you do not coat the edge your coating will eventually begin to peel at the edge. The easiest method is probably to mask off the contacts for the NFC antenna and spray the entire cover, inside and out. That way there is no rubber seam to peel at the edge.
And, if you decide later that you want to resell the phone, the PlasiDip coating is easily removable. Naturally, you can use this paint on other devices too.