I've read Kathleen Meyer's book "How to **** in the Woods".
It's obviously based on north American experiences, but there's a lot of valuable information for the rest of us.
Re the lack of T.P. (as she calls it), she suggests using either lots of leaves or, like Ed earlier, using water and your left hand.
When it comes to washing your hands afterwards, she mentions using an old bean tin with a string handle. Punch a hole in the side of the tin near the base and scoop clean water out of a bucket in to the tin using the string handle. Hang the tin on a suitable branch, or place on the edge of a flat rock, and wash your hands under the running stream of water coming from the hole in the tin.
It's a book that's well worth getting.