When playing hockey, always wear a helmet with a visor or a cage.
A shoulder to the mouth, a stick to the head, and a stick blade to the eye (thankfully and luckily eyelid.) have taught me that.
Do not borrow your cousin’s bicycle with the bent rear wheel.
Do not ride said bicycle at unsafe speed.
Do not ride the bicycle at unsafe speed on a gravel road.
Don’t be surprised when logical consequences arise from your actions. It’s undignified.
Never trust anyone who habitually calls random coworkers “dear”, “honey”, “sweety”, etc…
You have no-one to blame but yourself. Learn the lesson. Forgive yourself. Move on.
To expand on this: if there are helmets made for whatever activity you do, get one and wear it.
Check which parts of a machine are hot before you have a chance to grab one instinctively. The exhaust on a rototiller is on the top of the engine and will give you a blister that will last for weeks.
There is no way to tell the difference between cold glass and hot glass by looking.
No matter how quickly you use memory after freeing it, your program will eventually crash.
Washing your car to make it rain will not work. Leaving your convertible top down, though, is a pretty sure thing.
Only start one home improvement job at a time.
Make sure you double check how tight the nuts are on the motorcycle forks before you get to speed on the race track after your idiot of a friend adjusted them.
In tools you get what you pay for.
Beer+eBay+credit card=bad things. Hell, beer+credit card+internet=bad things.
Never fry bacon nekkid. (not a real life example I just like saying it.)
If you’re female, always have a ‘back-up’ tampon stashed in your purse somewhere. Don’t end up like I did, prowling Montreal at 6am on a Sunday looking for an open shop.
-That ‘gut-feeling’ you sometimes get is there for a REASON.
-A four-year-old will say “why” more times than an owl can say “who”.
-Don’t drink a soda with a mouth full of “pixie-stix”.
-If you hear a diaper-wearing infant give off a watery-sounding fart, wait until you hear TWO more before removing that diaper.
You cannot change the behaviour of others, you can only change your reaction to it.
Washing up liquid will not make a good substitute for dishwasher powder in an emergency.
Don’t put off calling for a Doctor when you are feeling very ill.
Sometimes even your instincts can be wrong.
My life got so much easier and my thinking so much clearer the day I figured *that * one out.
I can vouch that this is good advice.
If you’re in college, you’re not ready to get married.
If you’re not ready to get married, you’re not ready to have kids.
If you’re not ready to have kids, birth control is really important. More important than sex.
Probably one of the biggest revelations I’ve ever had. 
Don’t TOUCH any game that doesn’t have decent Codebreaker codes. They are all nightmarishly difficult abominations that will have you screaming for codes.
Whenever an entertainment company of any kind makes a mildly controversial decision, exactly one facet of the screaming psycho lunatic fringe will have a stranglehold on the discussions about said decision. (see: D, Initial)
If the regulars on a message board can get worked up in an otherworldly lather about Dance Dance frickin’ Revolution, asking them for advice about anything is a fool’s errand.
If your bicycle can’t go two lousy months without springing a flat or misaligning a wheel, it’s a sign that you should stop riding bicycles.
Lousy-paying jobs that take place at odd hours and require you to stand or walk for hours at a stretch also have ridiculous rules or production quotas, so you won’t last whether you want to or not.
Never take the mainstream news at face value. These guys are under more political pressure than most mayors and don’t even try to be objective anymore.
If a car develops recurring battery problems, there is one correct course of action: Replace the battery. Don’t clean the ports. Don’t test the alternator. Don’t replace this wire or tighten that connection or some other mincing little half-measure. REPLACE. THE. BATTERY.
If you cannot last at least twice the length of a standard movie without going to the restroom, forget about seeing movies in theaters.
Reality shows are the most bogus form of competition in existence, so there really isn’t much point in getting all worked up over who wins. Much less making predictions.
There is no end to human stupidity. Truly, it is the only thing I am absolutely sure is infinite.
Never try to talk someone out of an irrational belief. If someone has an irrational belief, using logic to show them that the belief is not true will never work, and will lead only to your own mounting frustration, and possibly anger on both sides.
If something is truly beyond your control, worrying about it is pointless and destructive. (And if I stuck strictly to the OP, I couldn’t post this one because I still haven’t really learned it.)
You can’t change the past. You can’t. You really can’t. No, no matter how much you might want to, you can’t change the past. Nope, sorry, the past has happened, and it’s unchangable. You can’t change it. Once something has happened, you can’t change it. Even if it’s just one second ago, you can’t change it. If you say something stupid, you’ve said it and for all eternity it will remain said. If you make a mistake, it remains made for the rest of time. If you wish you hadn’t done it, that’s too bad, you still did it. You can’t change the past.
And a corollary: Regret is only useful if it helps you to avoid a similar mistake. By itself, regret is pointless and destructive. (I’m closer to actually learning this one, but still working on it.)
Yup, I just learned that one too.
Second important lesson, the little red button beneath the cashier’s desk should never be pushed unless someone is actually robbing the bank. Otherwise you very quickly assemble a group of heavily armed policemen outside the bank and your Dad has to explain that the bank isn’t being robbed at 7pm, it was just his 14 year old son messing around.
Huh? What exactly are you describing?
I guess that puts the kibosh on my intention, when next re-entering the U.S., to say, “I have nothing to declare but my brilliance!”