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Trust each other. You may find it hard in life and think that it is impossible to keep your best friend happy. To be a good best friend really doesn't take much. All you have to do really is make sure you can both trust each other about really important matters. Don't try to trick or use your friend to your advantage; you need to make them know they can trust you.
Realize that your best friend may have other friends. Trust that you're really important to him or her and let your best friend have a social life outside of you. Love between friends is never jealous.
To prevent loneliness from biting you, have a small circle of friends. That way, if one friend is unavailable or he/she turns out to be your enemy, you'll have other friends to support you. It's good to know as many people as possible, but only have a few close friends that you tell your secrets to.
Don't keep secrets. Be open about what's going on in your life and what you hear about other people. If you don't want to talk about something, don't bring it up in the first place. If your best friend insists on hearing and you still don't want to talk about it, say: "You know you're my best friend; if I were going to tell anybody, it would be you. But I'm just not comfortable talking about it with anyone. I promise you'll be the first to know when I'm ready talk, okay?"
Know that you'll probably go through rough patches. Bear in mind though that sometimes you will have to give them some space so that they can have time alone and think things through. Being a best friend means understanding when a little bit of time alone or space from them can be a good thing.