Something that I find useful for a toilet paper substitute is to take wipes that I have dried out ahead of time (reduces weight) and then re-wet them once on the trail. Make sure you use treated water for this too! You can introduce parasites to your body at either end. It takes way less material using this method than trying to use dry toilet paper-and since they contain some cotton, they hold up to being wet much better.

I guess what I'm trying to say is, I can get by on two or three wipes with what would normally take several feet of toilet paper. It is much cleaner too!