I usually don't go down on women anymore because they can't reciprocate very well, at least the ones I've been with. And yeah, I tell them what to do if they aren't "getting it" and they just keep on doing their thing.

"Hey, watch the teeth please?"
"Watch your teeth, they're hurting me"
"Get out"

I bled once from a bad BJ, too bad for her, she was the sweetest tasting girl I'd ever been with. If she knew how to give head we'd probably be 69ing it forever.