I have 16 and 14 year old girls.......
Cell phone = life. The prospect of losing said cell phone (and texting) = the ultimate form of punishment at this age. DD #1 gets her license in 2 weeks so driving my car will become my NEW form of incentive/punishment!!
Don't be surprised if the circle of friends starts to change a little/a lot. Girls tend to gravitate towards other peer groups in the 8th - 10th grades and generally happens slowly as they are around new people and get to know new people. Be cautious of a sudden shift in peer groups. Also be cautious of a sudden urgency on their part to get permission to do something or go somewhere with a new friend that's out of her "normal" pattern. Trust your gut and that nagging voice that something's up.
Don't be afraid to call other parents and make a point of letting your child see you talking to/meeting their new friends parents!!! I'm the parent who embarrasses my girls because I call to make sure that there will be a parent home, not an older sibling left behind to babysit! Don't get me started on a few hard lessons I've learned there.......
You'll notice a desire for more independence and withdrawl from you. It's normal and it's OK. I told my girls that with independence comes responsibility on their part (calling & checking in when I ask, keeping me updated on moving from point A to point B, calling from a HOUSE PHONE [not a cell phone - - very important in order to get a land line #] from where they say they are going. No follow through on their part = increased restriction & monitoring on my part.
As you start increasing your DD's freedoms and she says she's going somewhere with friends for a little while (a public place such as a restaurant, Starbucks, etc.), "accidentally" show up at said place, say hi to her and her friends, do your business and leave. Sneaky I know...but my girls got the message fast that I was checking to see if they were where they said they'd be and were with who they said they would be with. Trust needs to be earned and if I need to know I can BELIEVE what they are telling me. I've only had to do this a few times.
Don't ever minimize their problems....everything is black/white, right/wrong, yes/no......it's hard to find middle ground sometimes so take a deep breath and give it your best shot. We're dumb as dirt in their minds and they think they're the ONLY ones going through this and we're unreasonable, mean, and don't understand. Girls are very adept at using their words and tone to get in sharp jabs.
Be supportive and keep her so busy doing constructive things she wants to do that she doesn't have time to get into trouble or be tempted.
Above all else....make sure she knows you love her and will always be there to support her no matter what.