Never do anything permanent to my body, unless it is medically necessary.
People come in and out of life... Let them.
Do not date at work.
Under absolutely no circumstance do I mess with people in relationships.
Friends that are dating:
If they break up and I'm interested in her... I give one month per year of time, one month minimum, before I approach her. That helps prevent hurt feelings, it also prevents being the "rebound". Although, one time it cost me a possible relationship as she broke up with her fiancee' and immediately approached me. Unfortunately, she never came out an positively stated they were broken up and it caused quite a bit of awkwardness on my part. She ended up hating my guts, but honestly I probably just dodged a bullet.
Give yourself the respect you deserve. Corollary: Behave in a manner that you can respect.
There are more, but this is what I have time to type out right now.