The brilliant and bawdy 16th century satirist, Rabelais, in his hilarious Adventures of Gargantua and Pantagruel , devoted a whole chapter to this question and concluded that 'a well-downed goose' was the most efficient wiping agent.

Perhaps, in the absence of a goose, a furry bunny? <img src="/forums/images/graemlins/grin.gif" alt="" />