I run caged Shimano SPDs (M454s) on my mountain bike and I find that not only are they easier to relocate when you're trying to clip in but you've also got a bit of a platform for if you can't clip in (not great but at least as good as those one sided plastic things).

I also have a copy of the Brian Lopes mountain bike book where he talks about pedalling technique for riding clipless. I think often when you go clipless you just assume that it'll sort out your pedalling technique for you but the book gives a really good insight into how to put down lots of extra power and some great exercises to improve. Well worth a look IMO.