I think the biggest mistake beginners make with clipless is they don't unclip forcefully enough. I agree with setting the spring tension loosely (if it's adjustable on your pedals), but the main thing is, don't be shy about unclipping. Unclip with gusto when you stop! I've heard people say, after a fall, "well I think I tried to unclip, but I just didn't get my foot out in time." I'll say this, too: like most everything else, repetition is the key. Just keep on doing it, and eventually you'll have a hard time remembering the last time you fell. I fell a few times at first, but I can honestly say I haven't toppled over because of a clipless "incident" in several years.