Back up
everything of yours at least once daily. Use a cronjob so that it won't matter if you can remember or not. I don't trust the 'Cloud' after several disasters involving DropBox when travelling out of the country.
I treat my data like I treat my gold: "If it isn't under your own personal control, you don't own it at all".
Store your files such that you are not backing up unnecessary stuff - such as the system itself because that's what system re-installs are for.(Apart from your various config files which are included in the daily backups)
I divide my stuff into four main categories: config files and installed-package lists, databases, personal stuff, and "write once, keep forever" archives.
The config-files and installed-package lists allow you to reinstall your system itself from scratch in less than an hour, in most cases.
Databases are self explanatory. Dump the databases at least once daily, reload it when your system crashes.
Personal /home directory stuff is restricted to personal documents and stuff that I am currently programming. That is it. Nothing else. What else would you need? That small amount is generally held down to less than 10 gigs.
For all else, (ebooks, videos, photos, music, etc, etc, etc) there is the archive directory. That is stuff that needs to be kept, but isn't actually private and personal.
The first three categories are included in my main daily backup as a full snapshot.
The last category, the archive directory, is backed up as rsync changes. This goes to several separate backup media. On many days, there will be no changes at all and the rsync backup takes less than a couple of minutes.