Talking is probably the largest factor I can think of.
To keep the other person interested in a conversation don't talk about yourself too much. Ask questions to get to know the person better, talk about any interests you have, classes, guys/girls, parents, etc. Becoming genuinely interested in the other person.
A somewhat easy way to get a conversation started is to make a short complaint about something that you know the other person doesn't like. Generally the other person will complain too and you'll have you're conversation. You have to be Very careful not to do this often because you don't want the other person to think you're a whiner.
Pay Attention to what the other person says! This one is Extremely useful as you can bring it up at a later time and the other person will be surprised and pleased that you remembered.
Continue to speak to everyone you know regularly. If you are walking and see someone you haven’t talked to in a while, stop for a moment and talk to them. It does not have to be a long conversation, just say hello, see how they are doing and if anything is wrong. You would be surprised by how little effort is required to win the friendship of others when you require nothing of them.
Be nice and generally try to end the conversation on a good note. You don't have to make sure the other person has a great time Every Single conversation, but you don't want to think of all the petty arguments they've had with you.
Arguments... Don't. There are VERY few 'good' arguments. "The only way to get the best of an argument is to avoid it."
If you're wrong, admit it.
DO NOT criticize the person, or a friend, when talking to the person. If you want to point out something they did wrong, try to point out something similar you did wrong.
Smile! Not a forced smile, the person you're talking to will be able to tell the difference. And you don't have to be an overly happy hyper-active person. Just smile naturally and, if you can, be somewhat happy. Remembering good times with your friends/family can help.
Let the other person do a lot of the talking and be a good listener. (Somewhat stated in my first advice, ask questions, talk about interests the other person has. Answer questions sincerely.)
Honestly make the person you're talking to feel important. You don't want to fake this one or do it too often, but if you honestly think they did something well, give them that praise.
If you want more advice, there are a couple good books that Will actually help. How to Win Friends and Influence People, is probably one of the best ones. It's an old book, but trust me, it's just as useful today as it was when it was written.