If you are a waitress, don't flirt with the married men!!! *ETA*
If you're trying to get a better tip, then do your f*cking job in a professional manner! If you have to flirt because you suck at your job, then learn how to do your job better!
*Edit* Since I'm being accused of having no self-confidence and finding a waitress a threat (lol), I'm editing this to say I'm talking about blatant flirting and ignoring the spouse/partner. Touching, ignoring the rest of the table, giggling, talking in a lower voice so only he can hear; all that is RUDE, disrespectful, and least of all, unprofessional.
This doesn't happen every time we go out, but it has happened a few times. As I've said before, if you do your job well and have a nice, friendly personality, you shouldn't have to resort to flirting just for a larger tip.