I have Look clipless on my road bike, plain ol' spds on my road bike, I have been riding clipless for years on my roadbike and a while on my Mtb, and I still fall over! I fall over regularly on the start of most rallies and organized rides, because I tempt fate by clipping in while going at a snail's pace.
On my MTB, I keep the tension quite loose so that I can clip out quickly if I need to put a foot down. I have learned the uber-cool look of grabbing a tree and staying clipped in if I am waiting for people to catch up or we are resting...but I have also lost all my uber-cool when I push off and am in too tough a gear and fall over.
I like the SPDs on my mtb much better than the Looks for ease of clipping in and out...and of course, the SPDS are set up to be easy. The Looks are great while I am pedaling, but if I am riding in traffic it is a pain.
Keep your elbows in, tuck your chin and fall if you are going to fall. Then look up, grin, and hopefully someone is grinning back at you! The worst is when everyone is doing that "look-away-we- saw-nothing" thing.