1. Show it off. An 18 year old body and skin does not last forever. Enjoy it. All those things you think you should be self-conscious about will be nothing in 10 years. (This is the same thing I tell myself now so I don't look back when I'm middle aged and think "what was that under 30 year old so fucking worried about?").
2. Don't allow huge weight fluctuations. No one is saying be super thin, but going up and down on the scale so often will leave you with stretch marks that cannot be repaired. Also (Duh) it's just not healthy. Keeping a steady, healthy weight is super hard but you need to at least try.
3. Stop sunbathing. That tan looks awesome now but you will have sun damage like whoa. Skin looks older than it should, wrinkles will start younger, and sun spots will be hard to get rid of without lasers. Trust me, I went to the dermo yesterday. Insurance does not cover your sun spots.
4. Spider veins will happen (it's in your genes) so don't cross your legs. Keep legs elevated when relaxing, and hope for the best. Anything that helps good circulation is a good thing.
5. Go big or go home. Tattoos are nothing to regret, they are a time in your life that you are commemorating, a moment you won't get back, even if it's a silly one. You do however want to listen to your tattoo artist's advice and go bigger. It will age better. You will wish you had started bigger and want to add and add and add. Oh and guess what, tattoos will be super mainstream soon so don't be so afraid of having them show.
6. Thrift stores are going to be cool one day. All that vintage stuff from your parents: keep it. Stop cutting it up and trying to make it modern. You will wish you had kept them in tact soon enough.
7. If something looks bad on a model, don't by it. 99% of the time it will look just as bad, probably worse on you. It's a good idea to try stuff on even though you hate going in fitting rooms or even real stores. If this isn't possible, it's good to make sure clothes aren't final sale. You will end up with some great designer stuff you will never wear cluttering up your closet because it is final sale and was just too irresistible to pass up online.
8. Don't "save" your good, expensive, or favorite clothes. Wear them! One day they probably won't be cool anyways, so wear them, love them, now. Also, what is the point of having gorgeous things if they stay hidden in your closet. Same goes with expensive makeup. Use it, it will eventually go bad anyways.
9. Save money for laser hair removal. Soon enough those lasers will be advanced enough to work on lighter hair, darker skin, just about everyone. Shaving is a pain in the ass. Do it young before you have a mortgage and before something cosmetic like not having to shave your armpits will seem like a luxury you can't afford.
10. Don't worry so much, your almost 30 year old self turns out fine. Be weird if you want to. When you get older fitting in will seem boring. Enjoy the present, you only get to be young once.