Yesterday in aikido, Scott Weaver had us play with an interesting ukemi exercise that helps introduce a bit of a random element to our practice.
When we're first learning ukemi, we typically do it with conscious effort: we do it along side someone else, with a class, and to a count, etc. as part of our warm-up. Then, once we get enough repetitions, we can start playing with a few variations (rolling opposite hand and foot, etc.).
The last stage of ukemi development comes when it begins to sink into our subconscious mind. We fall when we don't expect to, and the proper ukemi just comes out. It's happened to me, and several people I know, out in the "real world" in situations having nothing to do with self defense. You slip, you step off of a curb you didn't see, slip off a soapy truck bed (not me, but a good story) and you hit the ground before you even realize what's happening. Other people get hurt or break things; trained budoka, however, more often get up, brush themselves off and go about their business.
But there's a barrier between falling as a result of a conscious decision and having something yank you out of your world unexpectedly and send you hurtling toward the ground without you worrying about what's happening. In a word, it's fear.
Millennia of evolution has taught us that falling is bad, that hitting the ground will get you hurt. At the very least, protect the brain and face even if that means an arm or wrist has to break in the process. It takes time and practice before the subconscious can ever get used to the idea that it's okay to fall down. I find that younger students adapt very, very quickly (part of the inherent fearlessness of youth), and older students take much longer—if ever—to get fully comfortable with falling. Something about an older mind resists the idea more than a young mind.
Much of that transition from conscious to subconscious will happen in the course of regular training. Even still, many times we're working on a specific technique, and we really know what fall is coming up, so it still isn't always 100% spontaneous.
Which brings me to my original thought: Scott's exercise. All you do is pick one person to stand in the middle of the mat; everyone else stands around him. The person who's "it" simply starts walking (if you're really comfortable with falling, you can even close your eyes). Everyone else will, one at a time, come up and do something to disturb his balance, to interrupt his gait. It's nothing drastic, we're not trying to "throw" the guy. Just a hand, something to alter the "norm".
The job for the guy in the middle is simply to go with it. To relax entirely, to go with the flow, absorb the energy. It could be a back fall, a side fall, a rolling breakfall which in turn could go in a number of directions. That's it, really. But it goes a long way to overcoming the fear and really lets ukemi become as natural as walking itself.