Any kind of sexual method, technique or activity that you are involved in, so long as it occurs on your roommate's futon without his/her prior knowledge and results in semipermanent evidence of said activity on said futon. (Whether accidental or not.)
This term is especially viable when used as though it were a safe-sex method.
"My roommate owes me a new futon. He used the Roommate's Futon Method with his girlfriend last night."