If only older me could have advised younger me that a pink knit tie and white pants might not entirely impress my colleagues at my first internship.
That's an extreme example, albeit a relatively harmless one, but most people would likely have benefited if their younger selves knew what experience has taught them. That's especially true in the workplace, where sometimes you have to make a mistake to learn from it, but in many cases, you would be better off if someone had just helped you avoid the error.
Nearly everyone has moments in their career they look back upon and wish they'd had the benefit of experience. Sometimes it's not even about doing something wrong -- it's just that you would have made different choices if you knew then what you know now.
Build (and maintain) a network
In my early 20s, I worked a job where I traveled a lot, meeting a lot of higher-ups at a variety of companies in the industry covered by the magazine I worked for. There were many dinners, meetings, and evenings over drinks with people I liked, and who appeared to like me.
Those were also people who could have been future employers, mentors, or even people who might someday need a favor from me. I made absolutely no effort to preserve those relationships when I left the field, so all the work I put in cultivating those friendships was lost.
Now, in the era of social media, that type of mistake is harder to forgive. Keep in touch with people and build a network. Help people generously and ask when you need advice, help, or even a job. Relationships matter, but they can be easily squandered.
Almost never say no
Younger folks, myself included, sometimes pass up opportunity because something better may come along. Even worse they may let an opportunity go because it requires making a change that involves leaving a friend or a significant other behind.
There's nothing I ever said yes to that I regret, even if it went disastrously wrong. Experiences are easier to have before you have a spouse, kids, and/or a mortgage. So dont' be afraid to go to that conference, take on that side-of-desk project, or accept that promotion that requires you to move. Try to say yes as often as you can, while it's still easy.
Happiness is worth more than money
While my career has mostly involved following my own passions or taking various flights of whimsy, there were a few times when I picked the wrong job because it paid more. Some of my friends made worse choices by pursuing careers because they paid well, not because they were inspired by the work. A rich lawyer or doctor, I have learned, is not happier than a social worker or teacher following his or her passion.
People say that if you follow your passion, money will come to you. Social workers or teachers may not feel the same way, but they might tell you that there's more to being happy than having a lot of money.
Be the best coworker possible
My younger self wanted to be the boss. I was willing to do anything ethical to get there, and I was not shy about showing that off. That made me a very good employee for my bosses, but not the best coworker.
It's true that promotions often go to the most visible, hardest worker. It's also true that many promotions don't go to the person who deserved the job or who would perform it best.
It's better to be a good coworker and build a base of support among your colleagues. Be there for them and support their careers. That's being a real leader, and it will eventually help you get where you want to go. In my case, when I started worrying about my coworkers more and myself less, that's when I became someone worthy of being in charge.
Now, as a wiser, more experienced person, I find a lot more joy in being part of a good team than I ever did in being the boss. That doesn't mean I won't lead a project or maybe even someday run a company, but that's no longer something I find necessary -- it's simply more fun to share success than hoard every bit of it for yourself.