"back them up back them up back them up"
Whilst I wouldn't for a moment wish to denigrate that good advice, it doesn't by itself address the issue of long term storage. In 20 years your backups are as likely to be unusable as your primary data.

Certainly keeping more than one copy of your archive data is a good idea, but it's also essential to copy it to new media from time to time - exactly the opposite of what's good for analogue data like tape recordings or print.