Another question I think will have different viewpoints based on your age.  A few of my favorite:

Courtesy of my Dad: Drive like everyone else on the road is an idiot. This has served me well in almost 20 years of driving.  I have never caused an accident, though a friend and I were rear ended once, and I have gotten a few speeding tickets, but I think remembering that I am not the only one on the road has helped me become a more defensive driver.

Treat others as you want to be treated.  This doesn’t always stop me from unleashing my temper on those around me, but I think remembering this makes it easier for me to apologize when I know I am wrong…and I am wrong often!

Love deeply and Love big.  I don’t think this is something I heard, more something I have learned.  You can’t love halfway, and you can’t love someone and be unwilling to show them.  Love with all your heart, and be honest about it.  Sometimes it goes your way and sometimes it doesn’t, but there is ALWAYS something to be learned.