Sawdust works very well, as I'm sure peat moss and ash do as well. A large quantity of ash will severely alkalize the resulting compost, which may be a serious concern depending on your soil's existing ph level. The best smelling outhouse I've used also incorporated a wide funnel on the front side of the toilet (when sitting,) which diverted the urine away from the solid waste. I did not ask where it went, although that would have been a good idea. This resulted in a much drier waste pile in their bucket, which reduced both the smell and the quantity of sawdust necessary after using the outhouse.