Best thing, from the fiber point of view. Try and find a soap free wool wash, boots the chemists is the one I prefer for washing all my spinning fibers and lambskins with, failing that asda do a really nice cashmere wash that really looks after your sheepskins. Dont use any softners or conditioners in the final rise or this makes the hooks at the end of each fiber lay flat and your snuggly lambskin turns into a skating rink and wont stay where you put it.
For stubborn stains and sweat marks, find an art supply store and get some synthrapol, an overnight tepid soak in a bucket with 2 or 3 drops will really loosen out all those stains.