I've read many variations on this technique here. Placing your dick in a bag between the cushions of the sofa, or between your mattress and base, is one of the most popular ways of getting off hands free. Here's another variation to the basic method:
Get your jiffy bag (better than thin plastic ones which tend to cling to your lubed cock).
Now get a soft bath sponge and line the bottom of the bag with it so that there is some of the sponge left over the edge.
Lube your cock and slide it in. Make sure you're well lubed so the sponge doesn't chafe.
The sponge adds pressure on your cock on the very sensitive underside and also puts pressure on your balls as you thrust.
You can go for a long time as your cock gets so sensitive that you are almost in a constant state of pre orgasm. It's one of the best hands free methods you'll find. Personally I prefer the bed option as it's a better height for me to stretch into. I start off kneeling to get my dick in and get comfortable, but then ususally stretch out so I'm thrusting downwards into the bag with the sponge making it nice and tight and rubbing on my balls..
Cleanup is a matter of choice: a guy I know who uses this method prefers to leave his loads to build up. He likes the feel when he goes in again. I clean up every time.
Believe me it's a great sensation. Try it and let me know.