Stand up for yourself. Always. The world is full of people who survive by manipulating others to do their bidding. Learn to say “no” in a diplomatic manner, and exercise that power early and often. Otherwise your life will not be yours.
On that note, don’t believe what people on a message board on the internet tell you as absolutes. 
I’m with everyone else on this one.
I’ve made excuses time and time again to excuse what I have felt when I got these. Just recently I have washed my hands of a few people and few relationships because of this. Not only do I know it doesn’t feel right, I know there deceit in some form or level.
As many times as I have tried to dismiss it, it keeps coming back. I’m tired of living in doubt.
My advice - Just because they are older then you (teachers, parents, etc) doesn’t mean they are allowed to plan out your life for you. Don’t listen to anyone who talks down to you, tells you aren’t good enough or smart enough to follow your dreams. If you want something bad enough, you have the power to achieve anything.
While it is a wonderful feeling to have people believe in you, the power of believing in yourself is much greater and powerful.
Making the wrong decision isn’t as grievous as never making any decision at all.
Practical: Always use a cutting board. Seriously.
Even more important: If you’re feeling down, don’t believe the things you tell yourself. You’re not as fat, stupid, or ugly as you feel you are. You are what you believe you are, so tell yourself the good things. Everyone is worthwhile and everyone, even you, has someone who cares about them.
DO NOT stick your tongue on the ice-cold steel pipe, no matter how much you want to.
Absolutely! 
Don’t keep BenGay® and K-Y® in the same nightstand drawer.
You have been warned.
If you are having doubts on your wedding day, don’t go through with it. It’s easier to postpone/cancel the wedding than deal with a divorce.
Be careful about making something you love doing into your career. You may wake up and find one day that you hate it.
Always reassess your conclusions. Especially where people are involved.
Never cling to an idea too dearly, it will bite back.
Our prejudices often hold us back.
Listen to your conscience, it’s the only friend you can rely on.
Follow your heart, but don’t be ruled by it - your head must govern.
Won’t define Kobayashi, it’ll just make me look like a cynic, when I’m not…not really…
If you’re in love with what you think is a wonderful person and everyone else says WP is horrible, listen to everyone else.
If someone does something horrible and claims “You Made Me,” leave them in the dust.
Or how much your older sibling says it’s fun.
I can sympathize. My girlfriend has decided to stay with me despite her vague conviction that I’m a sometime cheater and occasional drug user. I’m innocent on both counts.
As my own gut instincts have shit for brains, I’ve settled on more analytical methods…
Never, ever marry on the rebound. Ever.
The mental affects the physical. Don’t spend more time worrying about something than working to improve it, and don’t let anxiety get to the point where it makes you physically ill.
When wearing a back pack or other strapped carrying device, always use both shoulder straps.
I made the mistake of wearing a heavy backpack on only one shoulder and gave my self eventual long term serious muscle spasm.
This reminds me of one of my dad’s axioms:
The way you get your girl is the way you’ll lose her. (IOW, if she’ll cheat on her man to be with you, she’ll cheat on you to be with someone else.)
Which is very true.
I’m still learning, but here’s my current advice:
Your environment will never change who you are. Never. If you are hoping it will, you’re in for very severe disappointments.
People are almost always going to be less angry at you than you imagine they will be.
Some exceptionally good advice here, many of which echo what I would have said. My own contribution:
Don’t listen to the opinions of people you don’t respect (that can include parents).
The scars are always noticed, think first, think twice, then think again.