I too have a 12 yr old DD and a 21 yr old DS - I have survived one teen so far or shall I say he survived.
My thoughts are not always popular but heck they work for us, so here goes:
Pick you battles.
If she wants to wear 1000 silly bands, why do you care? If her clothes are a mess or she looks ridiculous, why do you care?
If her room is uninhabitable by your standards but she is okay with it, honestly, why do you care?
To me, these are the battles I am not willing to fight.
Chore list states bathroom to be clean, it better be clean or the next time I hear "oh Mom, could you????" I either ignore the request or "forget" I heard it just like the kids do when I ask them to do something and they don't do it. This actually works really well and is a very quiet way to get the point across.
I refuse to argue, I am not going to have a debate with a snotty attitude. If they have a legitimate argument, I am willing to listen but it better be presented thoughtfully and with respect, not with sarcasm, snitty body language, eye rolling or anything else. I don't do that, I expect the same in return.
She is who she is, you can point out areas where she doesn't stand up for herself etc. but really only if she brings them up to you. If they don't bother her, then you are going to be hard pressed to get her to change her ways. If she does bring them up to you, think of a suggestion of what she could think about doing next time etc....
My favorite thing this summer occured with DD. She had done a few chores halfway. We were on our way to dropping her at a friends house who lives a couple of miles from us in the same housing development (disclaimer that I did not drop DD in the middle of nowhere).
I stopped about half way to the friends house, pulled over to the curb and said, "bye honey, have fun" she looks at me and says "But, Mom we are only halfway there"...... "exactly" I said, "you only did your chores halfway so I am only driving you halfway, out!"


the look on her face.....priceless!
Parenting as someone else on this board said a long time ago "is not for sissys"