I have a couple of mopeds whose tanks don't come off. When I clean them out (using phosphoric acid --i.e. automotive rust remover), I just hold the brake levers, pull the ped way onto the back wheel so that it's almost inverted (be careful of your tail light!) and have a shake-o-rama. I get somebody to balance the bike when I do the same thing on the front wheel.
It's definitely easier if the tank comes off, but you can get away with just shaking the machine.