Such awesome answers from great resources so I almost decided to not answer. I have 17 and 15 year old girls though so whenever I have the opportunity to give my thoughts with any chance of not getting an eye roll, I take it :) BTW, I'm just a mom, not an expert so revert back to the experts where anything I say differs!
Don't be surprised if they go from your loving sweet daughters to venom-spewing, face-painted, versions of your darling children. It shocked me at how quickly my oldest went from being mild-mannered, sweet, and obedient to a hormone driven mommy hater. At least that's what it felt like. I was surprised at how early that happened. I didn't do it until I was 16/17. She did it at 13.
It helps to remember that you couldn't pay me enough money to go back to that age. It's just as hard on them as it is on us. Maybe even more so for them because we understand the hormone correlation. They just know their moods, thoughts, friends, bodies, everything is changing faster than they can acclimate. That knowledge kept them with a roof over their head after more than one arguement.
Stay ready for the moments when they revert..and they do.. to the sweet sane child. I never knew when it would hit, but made sure to stop everything when it did. A simple talk over making dinner was occasionally life-changing and eye opening and gave me insight into what a spectacular young woman was developing.
Know that they listen to you WAY more than they will ever let you know. They appreciate you giving them strong boundaries, enforcing them, but loving them through the inevitable mistakes.
Two of the biggest things I've taken from my teens that I'll go forward with to my next two (currently toddlers... who the heck thought THAT was a good idea!)
1. Teens/tweens are mentally big toddlers. True fact. The mistakes and consequences can be bigger but our tweens are made to push their boundaries and find their line just like when they were babies. It's their job. It's our job to do our best to make it as safe an environment as possible. It's our job to be their parent, not their friend and demand consequences with love. I wish I could just find a door latch or electrical socket protector that keeps away the drug/alcohol/sex temptation but they don't seem to make them. Let them know what is and is not okay and that there is always a consequence but that when they screw up you're always there for them.
2. I don't want perfect children that never make mistakes or push the envelope or find their boundaries until they move out. Lord knows I have had sleepness nights, midnight runs to find out where my daughter is, etc. HOWEVER, she comes home to where it's relatively safe (expect from my lectures and chores). The mistakes she makes now, I'm here to help guide her through. I'm here to let her know that while there are consequences, nothing is forever and you can still get back on the right path. She is in a place with unconditional love and support. I will not hope for the unchallenging child.
All that said.... I have a friend with a daughter who NEVER had issues, NEVER turned into demon-girl (I say that with true love for demon girls BTW), and maintained a close relationship with her parents all through life. I have lots of friends but only one with a scenario like that :-).
Prepare to roll with it, go to bed worrying a few nights, and coming out on the other end with a couple of fabulous girls.