As a former bartender, I took my fair share of ribbing from customers, so when I screw around with bartenders and waitresses, its ok.
I like to go in to a restaurant and say something like " Do you have Michelob?" "Yes, We do." "Good...Uhm, I'll have a Bud light." By me not smiling at all, it makes them feel as if they've missed something.
Or I'll ask if they know how to make a Rum and Coke.