Check your disk space
Use the du and df commands to check your space.
Use du -s to get info only on directories.
Use the file utility to find out what types of files you’re looking at:
Clean up temporary space
Check /tmp and /var/tmp for files with old access times.
Subdirectories can be old; check their creation/modification times. Many, like /var/tmp/packagename, were used during installations, and can be deleted.
Clean up spool files
Mail is likely valid to keep, regardless of age.
Most programs clean up after themselves, but you should still check! See the cups and samba directories.
Clean up log files
Check /var/log. Very old files may indicate misconfigured log rotation or failing cron jobs, among other issues.
Check how many kernel versions you have in /boot
This partition is usually quite small. You CAN fill it up!
Get rid of old package files
Have you downloaded .rpm or .deb files? Chances are you don’t need them. Frankly, if you do a reinstallation, you probably want to update to the latest version anyway.
Get rid of old build files
Be aware of where you’re building when you install. If you’re installing as root, you likely have expanded tarballs or build directories under /root.
Fire up YaST, APT, Yumex or whatever package manager your system uses, and take a hard look at what’s installed.