I've had this happen to me many times.
I would usually say something C+F like "Gee, I was going to ask you to buy ME one.
" or something like that.
However those responses didn't really get me anywhere.
What does work though is this.
After she asks you to buy her a drink ask her for her name. Then say, "Sure! I'll get you a drink come follow me." If you're extra smooth you can take her hand to ensure she follows along. Pick a random chump/stranger who is sitting at a table and take her over to him. With her behind you, quietly introduce yourself to the guy and ask him his name.
Then say, "Well Chico good to meet you. I just came over to tell you that my friend Sally is dying to meet you." Turn around and present Sally to him. Put your hand behind her back and literally direct her in front of the guy.
Then make your introductions. "Chico, this is Sally. Sally, meet Chico. Sally is a very fun and adventurous gal and I thought you two should meet."
Then, just smile at Sally, quietly* wish her luck with that drink, and get the hell outta there. (
*I say "quietly" cuz you don't really want the guy to know what you're up to. You have no beef with this dude so why be an arsehole. Anyway,
I've used this twice. Both times the chicks came looking for me later and wouldn't leave me alone. Eventually they were the ones buying ME drinks.
The reason this works is because hot chicks are used to chumps doing shyt for them all the time. It puts you above those guys and demonstrates confidence, cleverness, ****iness, and a sense of humour.
Some may argue that doing this might result in you losing the girl to Chico. What-the-fawk-ever! You don't really want a chick like this anyway. She's rude and a user! Your goal with these types is to f*ck 'em and catch a sudden case of amnesia.
Good luck.