Start by saying "At least." As in, "At least I won't have to see this rude waiter ever again." It shifts perspective from problem to solution.
Actively decide not to pick a fight. Say you have a dollar's worth of energy to spend every day. Is this person worth 80 cents of that energy? In the grand scheme, unless the situation is life-changing (like a maniacal boss, in which case you might consider a new job), taking deep breaths and letting it go is a wise choice.