My favorite was told to me by my mom when I got engaged:

Make sure you agree about sex and money. If you get both of those things right, your marriage will be happy. If either of them are wrong, you're in for a world of hurt.

Boy, was she right!

Gotta go with the classics:

1. Neither a borrower nor a lender be.

2. My treasure is not in this world.

My favorite is a bit different....

You can't take it with you!!!!