Wear Sunscreen.
The long term benefits of Sunscreen have been proven by science, whilst the rest of the advice dispensed in this thread has no source more reliable than our own, meandering experience. 
Come on, I can’t believe someone didn’t already post that!
Be happy with yourself.
Strive to improve but ultimately, being happy with who you are makes for a much better life than continually finding fault with yourself and trying to change.
eat your vegetables.
exercise regularly.
save and invest every nickel you possibly can… then save and invest a little more.
Question leftist authority.
Understand that liars come from all walks of life.
Garbage …in …garbage can. Hmmm…makes sense.
Never start smoking.
Don’t bitch and complain about politicians if you don’t exercise your right to vote.
Do not obtain a degree or follow a career path for the sole purpose of making a lot of money; follow your passion and you will end up a lot happier.
Distrust all people with an accent. If you’re not sure it’s an accent- distrust them just to be safe.
There is never anything too trivial to call Homeland Security to report. If they don’t take action, then whoever you spoke to last is part of the conspiracy and must be reported.
On the other hand, picking and squeezing at a developing zit doesn’t make it go away any faster. As amazingly difficult as it is, JUST LEAVE IT ALONE UNTIL IT’S READY.
A> SHUT THE HELL UP
No one wants to listen to your motor mouth running nonstop about everything and anything. I don’t care what you think about so-and-so, I don’t care what you did yesterday, I don’t care to listen to you repeat all of that endlessly over your cell phone to other people either.
B> When someone gets mad at you, they are not evil and they aren’t your enemy. Trying to turn the tables and shame or punish them for their anger rather than address the issue makes you the evil one.
I can’t stand how many people think that allowing themselves to get hostile, angry and even unethical just because someone else was angry at them is perfectly OK, while the other person is clearly in the wrong. Recognize your own anger, people.
Always…no, no…never, forget to check your references.
(I think the young people enjoy it when I ‘get down’ verbally.)
Stranger
Learn to change a tire, your oil, and other basic maintenance for your car. Know which color what fluids are. That way, if you find yourself in the middle of the interstate with a broken-down vehicle leaking red fluid, assuming no wayward animal ended up in your engine, you’ve got a transmission problem on your hands. You may not be able to fix it yourself, but it will sure help prevent the local greasemonkey from trying to scam you when you’re 1000 miles from home.
Wait 6 months before you rush out to buy the newest widget/fashion. You’ll probably be able to get it on sale that way, and by that time you may have realized you didn’t really want or need it anyway. Pay cash whenever possible, only use CCs or get loans for the big, important stuff like repairs, cars, and houses. (This advice I really need to listen to myself… I tend to be able to eat more than Ramen noodles if I can control the impulse buying)
Adopt a stray, mentor a child, volunteer to help the elderly. Give back to the community. You’ll help someone in need, feel better about yourself, and it sure doesn’t hurt to build up a nice supply of good karma.
As others have said, vote.
Don’t let fear, setbacks, or disappointment prevent you from doing something you love, whether it’s a career, relationship, or realizing your dream to travel around the world. Evaluate your attitudes and make sure they’re not holding you back. As Henry Ford said, “Whether you think you can, or you think you can’t, you’re right.”
I almost always regret the things that I didn’t do, not things that I did. (Within moral and to some extent legal boundaries, of course.)
Always be prepared to accept the possible consequences of your actions, good or bad.
Never blame on malice what is adequately explained by stupidity. However, don’t discount the possibility that the malice does exist.
Don’t marry someone expecting them to change. Chances are, the bad traits you think your influence will smooth away will only become deeper with time – and your influence (however welcome it was at first) will only be resented.
Don’t marry young. You’ll do things in your 20s that you’ll never do the rest of your life. Take chances. Travel. Unless there are compelling reasons (your family needs you for income; sickness; death) take this time of your life to explore and learn.
Advice from those older than you really can be good advice. Be discerning, but also realize that the world wasn’t invented when you climbed aboard. Listen to the wisdom of your elders, for they too once were young.
These are things I learned the hard way, but fortunately God smiled upon me late in life and I’ve found my kindred spirit and true happiness.
As my grandmother used to say, “Please” and “Thank You” are always fashionable. You’d be amazed at what simply being polite to sales and service people can do for you. Really.
If you call a customer service number and get an incompetent boob, instead of making a fuss, just try hanging up and calling again. 9 times out of ten you’ll get a different person who can actually understand and help you.
Don’t buy a new car unless you are completely flush with cash. You’ve lost half your investment just driving it off the lot. Buy a used car and save your money.
When a snarky wisecrack or a harshly critical remark about someone springs into your brain, filter it through several layers of thought before uttering it or writing it. Even if it’s funny, it may be hurtful, and you may regret it for years. And the odds are that it’s not nearly as funny as you think it is, anyway.
> Never miss the opportunity to pee. Might be awhile 'til you see another bathroom.
I disagree with this one. I’d say that it’s 1 times out of 10 that you get a person, who speaks adequate enough English and can actually help you.
I usually just ask to speak to their supervisor.
Don’t make a habit of whining and complaining. If you are unhappy about something that you have control over, shut up and try to do something about it first. Don’t just sit there and gripe. Really, no one wants to hear about it, whatever it is.
Have at least one hobby or interest that has nothing to do with work or family obligations.
I can’t believe someone beat me to this. It’s amazing what you learn while living in NYC.
Also:
save up 2x your monthly living expenses as quickly as possible. Not in case you lose your job, and not just to have savings, but because it will keep you from feeling that pressure of living paycheck-to-paycheck when you’re young. That stress is terrible, and having some slack in you bank account will alleviate it.
Living with people is hard, plain and simple. Doesn’t matter if you’re dating, if you’re married, if you’re roommates, if you’re siblings, etc. Don’t judge each other too harshly.
Gore Vidal’s advice: Never pass up an opportunity to have sex with an attractive person, go to the bathroom, or be on television.
My advice: Before making any major purchase (I’ll say more than $200), buy a membership to Consumer Reports. They range from $5.95 (montly) to $26.95 (yearly). While they’ve had some criticism (mainly that the things they test for aren’t necessarily realistic [for example, in ranking car seats they were said to have used crash tests that no child would live through]) and while you might not be able to afford the very best, at least pay attention to which cars/appliances/computers/etc. scored worst.
With housing/computers, figure out the absolute most you can afford (that will leave you some breathing room) then spend all of it if necessary. With housing: if you’re not happy with where you live it’s going to affect every aspect of your life. With computers, you’re not going to be able to resell them for anything because in 2 years they’ll be majorly outdated, so buy the best you can that will last the longest.
Before buying a house or signing a lease, drive through the neighborhood at midnight on a weekend. (This has saved me from some big mistakes.)
With some grocery items/cleaning items/cosmetics/etc., the Dollar General brand or the store brand is a lot cheaper and works just as well. With some it’s best to pay more and buy store brand (aluminum foil, toothpaste, and garbage bags come instantly to mind). Learn which is which (and remember that many of the canned foods that are store-brand or off-brand are made by Del-Monte, Green Giant, and other “brand name” companies).
Your first loyalty is always to yourself and your family (family here being restricted to spouse-like-person and/or children). Your first loyalty is never to your employer. If a really great job comes along and you think you’ll be a lot happier there- take it without a second thought to whether or not it inconveniences your current employer. (The only exception is when your work history is too dotty, but even then- employers can be quite forgiving when hiring if you have the right skill set and persona.)
ABOUT LOOKING FOR JOBS:
NEVER be cute or gimmicky or conspiratorial with a résumé or cover letter- I’ve personally circular filed applications where the person led with “Don’t you want to do something truly great? Well hire me and you can!” or (actual letter) “Though my name might not sound it I am Hispanic, which means I’ll look great on your workplace demographic reports” or “If you’re not interested in me then please make copies of this and send them to other employers” (all of these were for a professional position).
Proofread every cover letter twice and then have a friend proof read it. You never want a typo in one.
And even if the job is washing dishes, better to arrive overdressed than underdressed so if you err do so on that side.
If you’re a guy, keep the following in your car if there is room and in a drawer in your office if there is not: a white dress shirt, a tie, a pair of slacks or khakis, underwear/undershirt, socks. That way if you ever stain your clothes you’re set for anything from casual to formal.
Find out as much as you can about your parents’ early life and childhood. Once they’re dead they take a library of information with them that you’ll never have access to again.
Never vote for a politician who refers to himself/herself as “The Education _____[governor, senator, etc.)_”
Recognize the difference between a personal prejudice and a legitimate concern.
Always listen to your gut hunch, especially where people are concerned, BUT at the same time always remember that your gut can be absolutely wrong and is mainly used for turning things into shit. Give the gut a vote but no more than logic and observation.
Rudeness is never called for. Anger and revenge are sometimes called for, but they’re different from rudeness.
Another Lazarus Long quote:“Go to hell!” or other insult direct is all the answer a snoopy question rates."
Sampiro’s variation on the above:
“Sometimes there’s no better or more concise and to the point response or comeback than Go fuck yourself, and it doesn’t necessarily make you a bad person for saying it now and then.” This counts as anger/revenge/self-defense, not rudeness, when called for.
Above all else remember that there is a reason why The Ethic of Reciprocity exists in all cultures and has for thousands of years.
Read mythology as well. You’ll be amazed how often it’s relevant.
Never forget a kindness and never forget treachery. Never give a traitor a second chance.
Last one and I’ll bold it because it’s probably the most important thing I’ve said and it took me many many years to learn it and my mother and many others never did:
LEARN TO FORGIVE. It’s for your own benefit, not that of the forgiven.
I don’t think forgetting is possible, but forgiving is, and it doesn’t mean you have to make the same mistake again or take the person back into your life, just get over your anger at them and move on. This is very very difficult but I can tell you from personal experience you’ll feel one hell of a lot better.