Trust

It can be hard to trust someone sometimes, especially when they have done something that broke it and shattered your trust in them.

Think about whether they were trustworthy at all, in the first place. Have they repeatedly lied to you? Broken or stolen something that you possessed? Are they a best friend of yours? If they have done these things, give them another chance. However, if you have caught them talking about you behind your back, or ignoring your existence, he/she may not be a good friend at all. Read this article for more information.
Treat others the way you would like to be treated. You wouldn't like it if someone was being mean to you. Rethink your actions. Have you done anything wrong? If so, read this for more information.
If you are the popular kid, you are probably subject to people who will want to be your friend because they will be friends with the popular person and will probably gain a small percentage of this popularity. See Tips to identify how to see if someone wants to truly be your friend, or just because of your position. Look at the oh-so-messed-up-lives of the stars that everyone used to adore. Britney Spears, Lindsay Lohan, Jamie Lynn Spears, and everyone else. They are so famous, they have so many fans, so much controversy, they have a pretty hard time seeing who they can trust, and who not to trust. Learn how to tell if you can trust somebody.
* If you can trust them, it will be obviously showing if they care about who you are and your well-being, or just about you, like the popularity scenario.

* Learn how to tell if someone is lying. If they are, it will show, depending on how much they tell the truth. Real liars will make up the most believable excuses, while beginners will be sweating and looking elsewhere to avoid the tension build-up. Observe the Adam's apple. Often, the Adam's apple will bob up and down (a sign of swallowing, another technique to avoid the tension). Read all about it here.

* If you detect lies all the time, then you will probably end up without friends. See this to cope with such situations.