An obvious one that comes to mind is "never pet a strange cat in a pet store that looks friendly". Ouch! found that one out the hard way.
If I had to choose one grand prize advice giver it would be. Never, ever stop talking to the people in your life that you love because you are not in agreement with their choices.
This is huge. I see so many people that I have known over the years, not be in contact with their child or parents or friend because they just did not like something that was said or done. Life is way too short. We all need to forgive and forget (more advice) and move on to the things that are really important.