I think a leather seat cover works best to prevent slipping.

If leather pants would make my butt look that good, I'd certainly wear them.

I'm trying out a 1/2" thick top pad made by PRO PAD Inc. It is a honey comb material that's advertised to increase under butt air flow. I got the pad to help with butt burn, but it would help with your issue.
Got mine through J&P Cycles.
http://www.jpcycles.com/product/504-603