"this too shall pass" - good, bad or indifferent, nothing lasts forever. Cherish the good, endure the bad & let the indifferent happen.
"treat others as you would like to be treated" - which is challenging when you are feeling very testy because the stupid phone system has hung upon you twice and you have been on hold for a total of over 40 minutes in an attempt to convince your satellite provider the receiver they gave you is a piece of crap. You end up muttering it like a mantra while having evil thoughts.
"don't slam the door on your way out" - you never know when you might have to go in & face everyone again later.