I try to keep it very tidy because otherwise I would create a mess.
"Dirty" and "Downloads" are working like /tmp, except they're getting wiped every 5 days [1].
Everything except "Downloads" and "Dirty" is backed up daily, incrementally and encrypted with s3cmd [2]. Some Inspiration here [3]. Sometimes I delete old stuff manually with s3cmd or from the S3 backend.
Dotfiles and bootstrap scripts are stored on Github and Dropbox, so are serious projects.
I also keep track of installed packages to be able to restore state on a new machine [4]. I'm not very emotional about restoring my machine. With a fast internet connection it takes me like 20-30 minutes to completely restore a wiped machine.
[1] http://askubuntu.com/a/20831 [2] http://s3tools.org/ [3] http://www.triatechnology.com/encrypted-incremental-backups-... [4] http://askubuntu.com/a/137991