What choice or act from your own life would you advise others against making?
For me, I went to college despite not having any desire to go, especially not wanting to go there, and not knowing what I wanted to study. I wasted a lot of years at a place that was a terrible fit for me (Notre Dame).
Even if something turned out okay, what would you tell people not to do if they can avoid it?
I’d advise against thinking that just because you are smart you can figure what investments will go up in value.
Get drunk.
Don’t go into college straight out of high school.
Have anything to do with the skank that was my second wife.
Get yourself into debt as a young adult. If your post-secondary schooling takes years longer, it’s worth it if you can do it without debt.
Research your careers better - I used to just jump into things without doing hardly any research (and that’s part of where my debt came from - paying for courses and diplomas that I ended up not using).
Smoke dope all day every day for about 15 years.
Don’t let yourself become obese. It’s extremely hard to get it back. Catch it early and do something about it.
That’s a good one - that reminds me of another one. Don’t let the things you were good at as a kid go by the wayside - it’s much harder to pick them up again as an adult. I used to be a fantastic skater as a kid - I can barely skate around a rink now.
Have a hemorrhoidectomy and fissurectomy. It was a month before I could say, “There’s a chair. I think I’ll sit down!” It was the most painful experience of my life. You can’t cough or sneeze. Having a BM is literally a screaming experience.
If I ever get another hemorrhoid I’ll tuck it in my sock and go on about my life.
Seriously.
Maybe the doctors have gotten better about this in the last 30 years. But maybe not. I wouldn’t chance it.
Be a single parent.
Motorcycles.
Stick it in the crazy.
Don’t major in Psychology.
never eat raspberries
A bit obvious, but don’t try that first cigarette.
I spent a decade smoking, and a decade getting off the addiction.
When someone is clearly trying to pick a fight, walk away… no matter if you could take him, just walk away.
Go fight in Viet Nam
I’ll echo jsgoddess and others’ advice not to go to college until you know that you want to go and have some idea of what you want to study. The only reason I don’t regret going into the army AFTER college is that I met my wife in college.
I’ll also second Opal. Your weight is like the ocean. It’s best not to turn your back on it. I’m coming back, though 
Don’t date emotionally unstable women. Not even if they’re really hot.
If you have kids, make sure they take school at least semi-seriously from the beginning. I find myself wishing that I hadn’t goofed off in high school so much. I mean, I wasn’t really goofing around, but high school was a living hell, and there was a lot of noise in my head, and a lot of things that should have gotten done wound up not getting done. There was a lot of wasted time. I did decently in college, but I could have done a lot better if I had prepared more in high school and just ignored the distractions and the idiots.
Don’t shy away from risks. I was always afraid to take them, and the first real risk I took was joining the army. I’m not saying it was the best, most well-thought-out decision I ever made, but it marked the beginning of a new life and a new relationship. If I had it to do over again, the number one thing I would have done in life would be to take more chances while I was young enough to do it.
Mentioned above, but I’ll second it:
Don’t EVER start smoking. EVER.
Don’t take benzodiazepines unless there is no other choice, and never let anyone ween you off of them too quickly.
Withdrawal is probably the closest thing on Earth to the Christian concept of Hell.
I also agree with the college thing. I also would recommend not becoming a music major if you have a scholarship that depends on high grades–unlike high school, you do not get graded for effort, but actual talent. If you want to become a music major, do it after you have a few years of regular classes under your belt.
Also, don’t become complacent with the easy As once you go back to a community college to finish out your basics, and lose your Pell Grant.
Oh, and dont’ miss the first day of class by accident, and then not talk to the teacher and get the syllabus. You can wind up failing a class that way.
Marry a lawyer.