At 40, hardly a ripe-old age, I've made a sufficient number of mistakes, and studied those who don't, long enough to offer a beta-version of my top pieces of advice ("Mine goes to eleven"). This is stuff I wish I knew 20 years ago (note: this list is also accessible via http://tinyurl.com/whatiwishiknew).
Friends who are friendly at their convenience are best reclassified as "acquaintances."
When choosing between two options, do what you will remember in five years.
Some battles aren't worth fighting (most of them aren't, actually).
Everyone lies, and usually not as well as they imagine.
"Hard working" almost always beats "smart."
You will lose every thing, not everything.
Pack only what you can carry alone.
Kindness is a lifeforce multiplier.
It's rarely as bad as you fear.
See the world.