If the bartender looks like she is having a really bad day, yet still looks me in the eye, and tries to smile, I usually leave several dollars for whatever I get.

If no eye contact is made or service is poor, I leave nothing.

I always bring holiday gifts to all the workers at my favorite and extremely competent SkyClub, SFO. There they also all seem to end up bartending at some point, so its sort of a yearly tip.