Brush your teeth. Not just right before dates but all the time. And use mouthwash. And mints.
I can't count the number of times I thought I was attracted to a guy, got within the breath radius, and WHOOOF. A whole evening's worth of flirtation undone in a single pungent blast. Now all I can think, no matter how good-looking or charming he is, is "if I had sex with him, I'd have to breathe that for like a half hour."