Take one of those little plastic baggies, like the kind you put a sandwich in, and put in some lotion or vaseline. Slip it over your erection and then kneel down at your mattress and slide it all between your mattress and box spring.
The pressure of the mattress will make it feel just like a vagina. Plus, you can hump it just like you would with a girl without using your hands. I just heard about this yesterday and tried it last night after everyone else was sleeping.
WOW! I've had sex like 10 times, and this felt so much like the real thing it blew my mind. I can't wait to do it again. Plus, cleanup was easy, as my cum was indide the baggy.
You'll love it, I promise.