Read the directions, even if you don't follow them.
Do not read beauty magazines. They will only make you feel ugly.
Understand that friends come and go, but with a precious few you should hold on. Work hard to bridge the gaps in geography and lifestyle, because the older you get, the more you need the people who knew you when you were young.
Live in New York City once, but leave before it makes you hard. Live in Northern California once, but leave before it makes you soft. Travel.
Accept certain inalienable truths: Prices will rise. Politicians will philander. You, too, will get old. And when you do, you'll fantasize that when you were young, prices were reasonable, politicians were noble and children respected their elders.
Respect your elders.
Don't expect anyone else to support you. Maybe you have a trust fund. Maybe you'll have a wealthy spouse. But you never know when either one might run out.
Don't mess too much with your hair or by the time you're 40 it will look 85.
Be careful whose advice you buy, but be patient with those who supply it. Advice is a form of nostalgia. Dispensing it is a way of fishing the past from the disposal, wiping it off, painting over the ugly parts and recycling it for more than it's worth.
But trust me on the sunscreen.