- Give your friend space
Understand if he/she wants to be alone or hang out with other people. Allow it to happen. There's no need to become clingy or needy. Friendship doesn't require that you always have to be paired together. Allowing one another the time to hang with other friends gives you much-needed breathing room, and allows you to come together fresh and appreciating each other even more.
- Never make a promise you know you can't keep
Good friendship is based on trust - if you break a friend's trust, the friendship may be very hard to salvage. Of course, if you have made a promise and planned to keep it, but circumstances beyond your control conspire to prevent it, let your friend know as soon as you find out. Don't wait until 15 minutes after you were supposed to arrive to call and say, "gee, I'm sorry." Instead, a quick call to say, "Hey, I know I promised to help you with whatever it is, but my mom is telling me we are going to my aunt's for the weekend, and leaving tomorrow just after school - that means I won't be able to make it. I'm so sorry. Can we reschedule?" That's just honoring the fact that your friend is counting on you, and respecting the fact that, given a little notice, your friend might just be able to get someone else to help with whatever it was - or not, whatever. But at least you won't be hanging your friend out to twist in the wind.
- Listen to them
you don't have to agree with them - just listen to what they have to say. Make sure they are talking too and you are not just running your mouth. Some people don't really find it interesting listening to someone talk about their feelings 24/7. If you're monopolizing every conversation with your feelings, they aren't getting anything out of the friendship. Invite them to share their hearts with you as often as you share yours with them.
- Don't be selfish
Grabbing, stealing and begging are big NOs in the rules of friendship. The friend will soon get tired of this and eventually more towards more selfless people who are willing to give what they get.
- Learn to share
As before, if you have a selfless friend, they will expect something back, even if they don't make it obvious. This does not necessarily mean giving them large expensive presents. This can just mean being there for them when they need you.
- Don't abuse their generosity or "wear out your welcome"
If your friend does something nice for you, then reciprocate. Money doesn't have to be an issue. Don't use your friends! Don't let them pay every time you go out, even if they offer. Don't help yourself to things at their house without asking, unless you are willing and they do the same at your house. No one wants to be friends with a moocher or feel used. If you borrow something from a friend, take good care of it and then return it without being asked. Also, if you end the friendship then you should return any gifts they bought for you, especially if they gave you any gifts under false pretences. It's proper etiquette.
- Live by the golden rule
Always treat a friend as you would want to be treated. If you don't there will be repercussions. Don't do or say anything to them that you wouldn't want done to you. Be there for them through thick and thin as long as they are a TRUE friend to you. Also learn to forgive, and apologize. Don't be a brat!