Accept each other for who you are.
Keep in mind that although the things you have in common may have brought you together, it's your differences that make your friendship so unique. For example, say you're into soccer, but she loves to paint. Don't try to convince each other that your interest is better; teach each other something about it and you'll both win.
Be honest with each other.
Telling each other how you feel can help you avoid silly fights.
Trust each other and your friendship.
If you're always honest with each other, this should be pretty easy. But there could be times when it becomes difficult. For example, if your friend blows you off in school, you may think she doesn't want to be buds anymore. Just trust! She may be having a bad day and doesn't want to bring you down. Or she may have a problem at home that's taking up all her attention. After school, give her a call and ask her what's up!
Listen to each other.
It's the only way you'll get to REALLY know each other. And sometimes, a girl just needs someone to let her blow off some steam.
Keep your promises.
Stick to your word. Simple as that.
Stick up for each other.
If you hear someone bad-mouthing your bud, let them know what she's really like. For instance, if you hear someone say your friend is snobby, point out that she's really just shy and has a hard time talking with people she doesn't know very well.
Don't talk about each other.
Even if you're really, really mad at her, don't spread rumors or bad-mouth her behind her back. When you make up, you'll feel bad, and if she finds out, that could be the end of the friendship. If you need to vent, try writing your feelings in a super-secret journal (you can always rip up the pages when you calm down).
Don't ditch each other for new friends.
Making new buds is great, but not at the expense of an established friendship. If your new friends want to hang out at the mall, ask if you can invite your bud. And if they say no, do you really want to be friends with them?
Even lifelong best friends are bound to argue sometimes. Just remember, if something's bugging you, you need to talk it out instead of letting it build up inside you. Be specific about the problem, and don't bring up every little thing that's ever gotten on your nerves.
Know how to say you're sorry.
Okay, so you guys had a major blow-out. It's never too late to fix things. Just give each other a little time to cool down. Once you've had a chance to think things over, one of you will have to make the first move (it shouldn't be the same person every time!). A simple phone call can help you work things out - and it doesn't even have to start with an apology. Just saying you feel bad that you're fighting gives you both an opening to sort things out. Who knows? By the end of the conversation, you may be feeling closer than ever.