My most important one is and always will be:
"If you've nothing nice to say, then don't say anything at all."
Nothing comes close to this quote for me. Words hurt. They really can take a wounded person and completely destroy them. Have I ever said hurtful things? Of course I have and I remember them with sadness that I could be so cruel. Swearing at my mother as a child, wishing someone dead in my teens, the bile I told myself as an insecure teenager, words that came out to hurt a girl at school who I had a long standing feud with..I'm not proud of a single one if those moments but they live with me with this quote because being nice costs nothing, and if you can't be nice, friendly, kind then just walk away.
"Let go of what is not yours graciously"
This is important to me because the quote really speaks to me when I miss people in my life that are just not there anymore. Mostly they are friends that I felt very close to and they have moved on with their friendships or entered relationships. They have chosen to move on and missing them hurts but they're not mine, they are their own person with their new circles and I have to accept that. I'm reminded to do it graciously because that stops your sadness turning to bitterness which is easy to do when you care very much for people and will probably never see them again.