Do you ever wonder why you're being ignored, or never being asked to do things? It's probably because you haven't worked on your personality.

[edit] Steps

Evaluate your current personality. Are you preppy, skater, punk, goth, rocker, ext. And whatever category you fit in, how are you liked? Do you like your current personality position? If you want to change from one to another you must think about these things.
Let people be themselves around you. If people can't be themselves, then neither can you.
Fit the part. Whether that means wearing more black, or popping your collar, you have to feel like that personality before you become it.
In any personality, be kind. Trust me on this one. If your going for that stuck-up preppy, hard-to-get kind of person, stop. Right now. People just think that's shallow, and they will not want to be around you.
Be laid back. Just go with the flow, and don't create a fuss.
Make sure you're in on all things cool. This means browsing magazines, listening to radio, watching TV etc.
Make sure you know a lot of people, preferably the popular ones. People will want to get to know you.

[edit] Tips

When you find your right personality, stick to it.
If you find you want to change your personality again, do it gradually.
Don't be the whiney wannabe professional victim who deludes him/herself into believing that s/he is persecuted by everybody, but accept it to yourself when people are attacking you and acknowledge the true scale on which it is happening
Just be who you feel like being - most other people's opinions shouldn't really have an influence on this, as they aren't going to bring you much pleasure in life
Use common sense to get along with people. If they are foul to you, avoid them.

Do not become something you're not, whether that means learning to skateboard, cutting off all your hair, or buying a new wardrobe etc.
Don't forget time for yourself
Don't even bother trying to change your personality if it's not evil or stupid - just go about your life and do that which you feel like doing, not the standard actions of the so-called 'safe' social norm that locks people into a cycle of mental illness and failure. If you're a teenager wanted to be liked more amongst peers, remember that the silly, destructive behaviour that people may inflict upon you will probably fade as these people get older and more mature
Be bold and defy the silly trends that are supposed to be non-conformist and those that are supposed to be conformist - develop your talents, question your beliefs and rise above the lesser beings who attack you for a feeling of security and ego gained from the belief that their 'safe' path is the best, when really they follow it because they are lazy idiots who refuse to make their lives better in the long term. If you're rejected, go about your everyday life without worrying about your reputation and silently sneer at the unpleasant people for failure and suffering - that's what humour is all about, after all!