I recently split about a cord of green elm with an 8lb maul. fjord has it right - avoid splitting across the rings as much as possible. My method for the large trunk sections was to noodle into quarters, then split off the point of each triangle (along the rings), then flake along the bark or, if you're feeling lucky, go for the cross-ring split. Crotches I don't even try, I noodle them into largish blocks. One thing I noticed in my elm was that the smaller limb sections split easy; it was the trunk that was the killer.