There was a bit in one of Ricky Gervais' shows, he was telling about how this kid used to wank by catching a fly, pulling it's wings off, getting in the bath with just the tip of his todger poking out of the water, then putting the fly on top of it. The wingless fly could do nothing but walk around which, according to this kid, was the best wank he ever had.
So yeah, try that!