It ain't pretty, but if I set up so that I'm sideways to a slope, I just shove stuff sacks, shoes, random stuff under the "downhill" side of the sleeping pad to sort of level it out..."
Just thinking about that makes me laugh...:)
I did this last year when a buddy and I packed in late one night. We could hear the wind really whistling on top of the ridge so we decided to stay 200 yards down the hill. It was pretty steep, probably 35-40 degrees and try as we might we could not find any elk/sheep beds to sleep in so we just improvised. He found a bush to wedge his bivy against and I used a rotten log on the down hill side...at 3am I awoke to find myself rolling down the hill 15 feet into a tree...It scared the crap out of me and pour Gary just about p!ssed his pants he was laughing so hard.
To do it again I think I would have used the paracord in my pack to secure the log to a couple of trees and it would have saved me the adventure...