I have two:
Way back when I was 16, I worked as a host at a busy restaurant, I would get really stressed when we had a long line at the door (the wait would easily get up to 1 hour on weekends), and I just started repeating, "you can only do what you can do, you can't do any more". As someone who has always really struggled with the need to please everyone all the time, it's really helpful when I'm running busy events (I work as an events manager now) or when anything is approaching FUBAR because of things beyond my control.
On a broader, life-changes perspective, I always loved a quote from The Riches (said by the actor now known as Suzy Eddie Izzard),
"Life's a river kid, you gotta go where it takes you."
Its helped a very risk-adverse me take some huge leaps and I've not regretted any of them.