One line that works, but only if you have been employed BY the bar, is "Good evening, I'm the house magician and don't worry, I actually DO work here." It usually gets a laugh and lets the table know that you actually work there and are not some psycho rando trying to horn in on their evening out. Also, note which tables, or people at the bar, have taken notice of you and are watching =- they will often be your next stop while making the rounds of the place.