Be a better sneak and a better liar. Don't think that if you always aim to do the right thing it will pay off. In fact, you dummy, trying to always do the right thing will be the very thing that creates the biggest problems. I'm not saying I'd tell myself to go around and wildly disregard laws and rules. I'm also not saying that I'd let my kids know when I disregarded some "right thing" , because I'd still want to be a good role model. I just know that being a better liar and bigger sneak can really pay off, and going around defending people for one reason or another either leads to some people thinking you're stupid or too weak or too nice.
I've always seeing not aiming to do the right and honest and "defending" thing as being weak and cowardly and selfish. Not every0one sees it that way.
I might also tell myself not to go around trying to make other people not-feel-bad about themselves. The very same people that we sometimes worry about not feeling bad about themselves can sometimes feel so good about themselves they're the first to jump at the chance to make someone else feel bad (or wish they could).
It occurs to me that I might like to tell myself not to respect and care about people enough that I'd keep some of their misdeeds/bad attitudes to myself (sometimes because I may see it no one else's business or sometimes because I think yap0ping about people to other people is a lousy thing to do). That one, though, I suppose I'd see as "miastakes I've made but wouldn't do any other way". (There's something to be said for people knowing they can trust you, so I'd still do things the same way when it comes to this one.)
Also, I'd tell myself that if I got a "whiff" of someone's "having some issue" with something I did or was, do a better job of keeping distance.