i had a roadie coach once who taught us how to crash effectively. as others have said here, the key is kind of ducking and rolling, trying to roll into it and out of it. sticking your arms out to stop yourself is somewhat instinctive, but it's also the key to broken bones.

i'm sure the knowledge doesn't hurt, and there are exceptions, but most crashes i've had, i haven't seen coming for more than a split second (or else you wouldn't crash, right?).