I just ordered a pair of these Coolmax socks that has a regular ankle sock and a built in cuff that folds over the top of your boot to keep debris from getting in. Much simpler that messing with gaiters - I wondered why I'd never seen it before. It doesn't clip to the bottom of the boot so it's just easier. Only downside I see is it's not waterproof, but it sure seems like a great idea.

Would it work for trail runners? I think Inov-8 has those mud socks that do something similar.

Cheapskates can do the same thing with the tops of worn-out socks.