This is such a great thread!! Some of my thoughts echo what others have said:
-Know yourself! And be okay with who's inside, even if you seem like a walking contradiction. Work to change what you don't like, but don't ever lie to yourself about who you are at any point in time. When you're self-aware, you won't make bad decisions and you'll have a much easier time understanding other people because you'll have something solid to compare them against. If I could give a person only one piece of advice, it would be this because if you work on this one, everything else will fall in line.
-Travel when you can! There's no better way to broaden your world view. And I've found (for some strange reason) that while on vacation I make major decisions about changes in my life that I implement once I get home. I think maybe it's something about the change of scenery that gives me the clarity to make up my mind about things. So, vacations are a good mental health tool
-Don't be afraid to be overly enthusiastic about everything, even the little things that no one else will understand. People admire enthusiasm and it's contagious. Along with this one goes...never grow up all the way. If you still like to play with Legos or skip down the street then just do it. And the third part of this is: never feel as though you have to give up childhood traditions just because you're grown and/or there are no kids around. You can still color and hunt Easter eggs, knock down a pinata for your birthday, leave milk and cookies for Santa. Fun and tradition are for everyone, not just kids!!
-When you start to feel lost about who you are, remember who you were. Over the years your interests and situations will change, but I've found that at the core people stay about the same (and to clarify, I believe all people are good at the core. They get labeled otherwise because of their actions). So, the chances are that however much you change throughout life, you'll always carry the same passions and ideals you had as a teenager or young adult (maybe even as a child) and those are what ground you. People are like trees: they need a solid grounding to soar upward.
-Learn to appreciate your family, even if you live at Dysfunction Junction

They are who they are and they have much to teach you if you let them.