My rules of thumb:
At least 3 different digital places (I have 4: PC, Cloud, DVD, External HD). I keep an HD at work, too
At least two physical copies:
paper printouts for passwords, bills, insurance, etc.
I wait for a sale and run photos through the lab and get physical prints.
I realized I don't know anybody's numbers. Although I can still remember my best friend's number from childhood.
Anyway I started wondering what I'd do if I was a ways from home with a brokedown car and dead cell phone. So I printed out my address book and keep a copy in each car and motorcycle. Don't know why it didn't occur to me before.