I rarely have a girl ask me OUTRIGHT to buy her a drink. Generally, they don't ask if they don't think you're an easy mark.
Sometimes I ask them to buy ME drinks..."Hey, what's that? Can I have a sip? Delicious...get me one!" Jokingly of course, but you'd be surprised how many girls i get to buy me drinks. IIRC, I've had pornstars buy me drinks. (then totally play me like a chump, but at least the drinks were free
)
Anyway...if she just comes up to you and asks out of nowhere, "Hey will you buy me/my friends a drink?", an outright "um, no" is probably not out of the question. It'd be like you going up to a girl and saying, "Let's make out".
The ol', "I don't even know you yet!" also works well. Humor is always good. "Not until we're married! What kind of guy do you think I am??" Dumb sh!t like that. Gets 'em talking.
I sometimes used to set up an alternating arrangement...you get one, I get the next, etc. If it's a DATE date, I'll pay for the date itself, maybe the first drink. After that, if she wants to be a lush and start taking shots, it's going on HER tab.
Umm...what else works? You could always make her "earn it" by getting her to pick out something good for both of you. I like fruity chick-drinks, so I'll tell her that if she can come up with something tasty, I'll get both of them. That way, for my money, I get a new drink to taste...and it challenges her to be interesting (not just good-looking) for it. Or if it's a crowded bar, get THEM to order the drinks, since t!tties get faster service.
I've tried telling her, "I'm only going to pay for a girl's drinks if she's coming home with me"...got called materialistic. Good way to get rid of drink-trollers but doesn't do much for "game".
Just remember...If she's really interested in you, not buying the drink isn't going to blow the deal, as long as you respond to it in a "cool" way. If she's not into you and she's just trying to get a free drink, there's little you can say that's gonna suddenly make her into you.
So whatever.
Whatever you do, have FUN with it...that's why you're out drinking in the first place. Some of you guys who come up with outright angry or ignorant responses...sheesh.