Make sure the people who see you have been listening to some happy music beforehand, even for 15 seconds.
Corollary: go to night clubs where upbeat music is playing -- '80s night is the obvious first choice -- rather than darker music. As a bonus, you'll get to see a bunch of college girls bouncing around in athletic shorts and over-the-calf socks, rather than a bunch of goths doing Tai Chi on the dance floor. (Anyone know when this became their preferred mode of dancing?)
This benefits guys, since you don't want girls to see you as sad or dark or too-tired, but it's especially helpful to girls. Guys can leverage some degree of dark moodiness into a cool, mysterious appeal, but no one wants to be around a mopey girl.