Im gonna tell you what i do. One of my friends asked me the same question, he followed my instructions and he was so excited with the success that he called me to meet up with them at the bar they were chilling at... so:
No need to tell them upfront that you are not paying for her stuff. Actually, you should pay... BUT charge back... with interest!!!
When i go out for dinner with a girl and the check comes, i use this line (my own material):
"I got no cash on me, so let me get this on my card... you got the drinks latter!". If she says "ok", ITS ON! You know she wants to spend more time with you, AND she will pay the drinks... HOW COOL IS THAT

. If she starts "oh no... bla bla bla" ... no problem... call the waitress and ask "innocently" if they can split the tab. Now, dont be affraid of telling her to get you a drink. In your case, not to be an ass, go like this "what do you usually drink?!... Uhmm... not my thing but well, make it 2 then, i got to go use the bathroom realy quick". This is kind of a shit test... see if she passes. Then just play around with the fact that she is paying you drinks. Role play as if she is your sugar moma or servent! Tell her she has bad taste for drinks and tell her to show you another drink. Pretend its normal for girls to pay you drinks so she doesnt feel awkward.
Another line i use when they ask me for a drink is this: "i only pay drinks to girls who pay back with sexual favors!" (wink and SMILE)... if she gives u a nasty look, quickly respond "so the next round better be on you or you will owe me big time!".
Or if you guys plan on going for dinner, tell her to think about a good icecream place to take YOU to... now she is in charge... she is supposed to pay YOU! Change roles! Dont be affraid of doing it! If she acts rude, then she might just not be a person you want to hang out with too oftern.