Well, K.....welcome to the world of teenagers!! I realize she is only 12, I'm guessing she just jumped to 13 mentally! lol My daughter is 15 now but take it from me...it was like I kissed my 12 year old daughter good night and woke up with a stranger the day she turned 13! She went through a really bad stage then. (Sometimes I'm not sure that it's over!lol!) My daughter had a really bad attitude and thought that she should have to do absolutely NOTHING! She went as far as to leave school early with a friend one day. I got the call from the principal and thought I was going to actually strangle my daughter when I found her! Please understand, my daughter had always been great...did her homework, got straight A's, honor roll every semester...so I'm not talking about some problem child from birth here!
I think that this is a hard age. They aren't looked at as adults yet but they don't consider themselves children either. Their brains aren't developed enough to truly understand the consequences of all of their actions yet. All that combined, I really think that their acting out is their way of trying to find their place in the world. If they come across as perfect little children, then their friends make fun and if they act out, their friends think they are cool, but the adults are shaking their heads thinking "problem teen"...and we as parents are left frustrated and embarrased because of such disrespect.
I wish I had the perfect answer for you. Here is how I handled it (and still do). Rather than taking away everything for what feels like an eternity, I take away the bigger things (t.v., radio, phone, going places), and depending on the "crime" I try to fit the punishment...so if she backtalks, I don't necessarily take away EVERYTHING but I might take away a couple of things and let her know that she basically has to earn them back. If she gets worse, I add to the punishment...taking away more privelages. I had to really stick to this. (If you take away everything and make it seem like it's going to last forever, they really have no reason to turn things around....what I'm saying is if she thinks that she's not going to be rewarded by doing better, why bother?)
Another thing I did was really started talking to her. I would explain why I was frustrated and why I felt let down and tried to keep an open line of communication. Things really started to turn around after I started doing all of those things together.
When she first started acting out I found myself frustrated, yelling and taking away every privelage I could think of. Then I saw most everything that I mentioned on some talk show and started trying to change how I was handling it and found that it really started to work. By no means am I implying that we never have problems! I guess that's just a part of being a parent!...to bad they don't come with instruction manuals! lol
Just keep in mind that it could always be worse! Good luck! :)