John Hawkins of Right Wing News blog wrote about 50 things that every 18-year-old should know. I wish I knew these two when I was younger, it would've saved me a lot of grief:
When men have a problem and they tell you about it, they want to know how to fix it. When women have a problem and they tell you about it, they just want you to listen.
If you get into a business deal with someone who goes to unusual lengths to convince you of how honest or Christian they are, watch your wallet and make sure you have an iron clad contract. They "doth protest too much."
What else would you add to the list?