I have found brutal honesty works well. i.e.
"Hey, Danielle right?"
She leaned out the window with a smile on her face and said "Yeah?"
Then I panicked and didn't know what to say and got all awkward. Oh the look on her face after that...
So, next trip through the drive-thru, if it were me, would go something like this:
"You know last time I came through here I was all set to flirt with you and then I saw you and your cuteness completely disrupted the connection between my brain and my mouth. I think I said something like, "Duh, blah, doblah." or something equally stupid. Just know it was not cause I'm stupid, but just cause you are incredible. Anyway, I'm going to go home and work on my flirting in front of a mirror and try again next time."