My daughter is only 3 but here are some of my thoughts:
I would wonder where she is picking up this behavior - my feeling is they have to learn it from somewhere, and I would be paying attention to what she is watching on TV. A lot of shows meant for tweens and teens may be technically G-rated, but can still how kids behaving like brats. I would even try to be aware of your own behavior and could she be somehow picking it up from you. Not blaming you in any way, just sometimes parents can be doing things and not even be aware that their kids are picking up on it (my husband's ex is like this). Does she have older sisters or cousins or friends from school that behave this way?
I would make sure she is aware of the specific behaviors you have an issue with - demonstrate them, if need be. Make a list and post it. And let her know that if they continue, there will be consequences. You might want to consider taking EVERYTHING out of her room except her bed and whatever clothes she needs for the day. Toys, TV, whatever. She needs to earn them back by demonstrating a better attitude. Another thought is to keep a sticker chart - good behavior earns smiley faces or stars and when she reaches a certain amount, she gets a special reward (a movie, rollerskating, whatever). Bad behavior earns her a frowny face and too many will mean a consequence of some kind.
I also like the idea of telling them, "Since you are choosing to do X, then you are choosing Y." Make her understand that she is choosing the consequences by choosing to act this way. "Because you are choosing to talk back, you are choosing not to have TV the rest of the day." "Because you are choosing to not cooperate, you are choosing not to have a play date with your friend." And stick with it - don't back down!
There is also a great book out there called "How to Talk So Your Kids Will Listen and How To Listen So Your Kids Will Talk." Check it out, and see if there is anything in there that will help you to communicate with your daughter better and hopefully get you guys on the same page. One of the things I like about it is it can get kids thinking about their behavior and how it's a problem, but also what solutions they themselves can come up with - making them take responsibility for their own actions and decisions.