I've recently become aware of ncdu, which stands for NCurses Disk Usage. It's a text-mode utility to explore disk usage; it's a bit like du, but more useful interactively. It can be pretty handy for figuring out where your disk space has gone.

It has a very powerful feature, with its --exclude-caches option. This ignores disk space in directories which contain a CACHEDIR.TAG file. If your backups also use these files, ncdu becomes a utility to see what's going to be backed up.

By throwing in a few more CACHEDIR.TAG files into cache directories (along with some things like save games for Steam games which are backed up to the Steam cloud separately), I've nearly halved the size of my personal backup set, which means backups are faster and more useful.