I'd tell myself to have fun more, lighten up (easier said than done at the time), explore more, and tell myself why. I'd tell myself that my dad was lying to me, and the results would cost me years and years of credit card debt.

Also - during the heat wave of '98, I'd tell myself to wait for traffic to die down before driving home with no AC in the car. It effed up my lungs pretty good, and that's permanent.