I've heard that in a rear-end accident, it's never the fault of the driver in front. (This may be an urban legend, but I have heard it.) Based on this, I've often fantasized about just slamming on my breaks and letting them plow into me. If they live, I'll take 'em for everything they've got.
Of course, I'd rather do this with a Mercedes crawling up my rear than a Ford Escort.
Fantasies aside, I usually just try to get out of the way. I always enjoy it when, a couple of minutes later, I pass the idiot as he's pulled over, getting a ticket...