How to turn someone down to pay for their coffee?
When she suggests that you accompany her, as soon as you two are far enough away from the others that no one can overhear, say something like "Will you be buying the coffee for us today? I think I bought coffee for us last time?" If that doesn't stop her from "hustling you" for coffee, then be direct "I think it would be better if you didn't ask. I'm starting to feel taken advantage of and I'd hate for a feeling like that to get in the way of our friendship."
Just dont take any money with you, then it doesnt matter How sweetly she asks, you cant pay. After you tell her a time or two you dont have any money on you, she'll stop asking
Just tell her that you simply don't have any money with you. Rummage around in your bag for a minute or two and say you've forgotten your purse/wallet. Or you could say, on the way into the shop and before she produces the change from her pocket, "Your turn to buy the coffees, I think." Or as you approach the store, tell her that you've brought your own water (show her a bottle) and say that you don't want a coffee so you'll wait outside. Good luck!
Just tell her "NO I CAN NOT, BUT THANKS FOR ASKING"... No other statement needed.
Given you want to change the dialogue between, consider something along the lines of telling your friend you are no longer able to buy the coffees every time you get together and suggest you alternate weeks so there is no pressure on one of you when you go out. Emphasize the time together is to be enjoyed, not concerned about personal budgets.
Others have given you good suggestions. I would only add that I think it helps her save face if your change in the usual script comes before the moment she asks you to pay. (She doesn't ask and then get turned down.) I don't mean if you get to that moment of her asking you can't say anything. Only that if you can screw up your courage to speak even earlier, that would be best. Also it should come before the coffees are ordered, because knowing you aren't going to pay might affect her choice to have coffee that morning.