If you had to give one piece of advice that is pretty much universally applicable, what would it be?
I wouldn't dare defile Douglas Adam's memory by not mentioning that you should keep a towel with you at all times, but my second contender is a surprisingly short three-parter:
Faking confidence means running your life as a con job.
It’s better to acknowledge when you feel insecure, find the root reason why, and then develop solutions to the unsolved problems giving you that lack of confidence.
For some people it’s as simple as their pelvis being out of whack. In some cases, all you need is a rolfing session.