With the age of MMA right now, everyone and their uncle knows how to pull off a rear naked choke. I even saw a video a while back where an elderly woman was robbed in an elevator by a guy who just snatched a rear naked choke on her, waited for her to go unconscious(takes about 5 to 6 seconds with a decent choke w/ panicing) and then robbed her.
This is a good defense to know about:
If all else fails, grab a handfull of balls or try this: