Mine has always been to treat people the way I want to be treated, as I believe what goes around, comes around and one day it will bite you in the bum if you don't treat people with some respect.
This is a two way thing though, so if I don't get it back, it kind of nullifies the motto for me and no longer applies.
What is your motto in life? What principles do you try to live by?
An example of this might be, if for what ever reason I had to let someone down at the last minute, which does sometimes happen, as there will always be outside influences in life. I would do my best to help the person I had let down to still be able to do what ever it was they were meant to be doing. I'd keep the lines of communication open and try and answer any questions they might have to fix the problem. I would never just walk away.