Actually, back in my high school we were taught how to do that muscle-clench resistance thing for when short on equipment. Thing is, I think that was more for exercising in general, and I dunno if that affects muscle mass or even strength much. It probably does, but I doubt it's as effective as having actual weights. I see no harm in trying it, though. Even slow muscle gain is good if it does help with that at all, if you're in no rush. At least it's good exercise overall. Remember that strength isn't just about muscle mass.