I would also agree with Caligurl and Ionic in that you may just not get on with the pedals. I used SPDs for a while on my MTB but I found that in order to be confident that I could get my feet out whenever I wanted I had to back the tension off so much that my feet would boing out on rocky descents, which is worse than a slow speed topple. Changing to Crank Bros Candys changed it all for me and I have them on all my bikes, including the roadie.

When I first put clipless on my MTB the bike shop advised me (which I ignored and learned the hard way instead) to find a grassy field (soft landing) and practise getting out of them quickly when riding really slowly and stopping suddenly.

I also think that technical offroad riding has made getting out of clipless second nature to me, much more than riding on the road, as you often don't get advance warning of when you might be unclipping you just have to do it instantaneously.