I helped a friend of mine figure something out without having to buy any toys.
Basically what we figured is to take a pair of panties, preferably satin as they provide the least amount of friction, and turn them inside out. He would lie them down on the bed flat and lie down on his side. Slip his cock between the front and back of the panties and using a 'humping' motion slide his cock against the fabric.
He loves it, says it's is favorite way to get himself off and I provided him with the panties necessary for it.