How do I free up disk space?

To delete downloaded packages (.deb) already installed (and no longer needed)

sudo apt-get clean

To remove all stored archives in your cache for packages that can not be downloaded anymore (thus packages that are no longer in the repository or that have a newer version in the repository).

sudo apt-get autoclean

To remove unnecessary packages (After uninstalling an app there could be packages you don't need anymore)

sudo apt-get autoremove

To delete old kernel versions

sudo apt-get remove --purge linux-image-X.X.XX-XX-generic

If you don't know which kernel version to remove

dpkg --get-selections | grep linux-image

Source: Limpiando Ubuntu: comandos y programas (actualización) (google translated)

• show top 10 biggest subdirs in the current dir.

  du -sk * | sort -nr | head -10
  

• use filelight or kDirStat to see where the disk space is going visually

• check if you have old kernels for deletion

  ls -lh /boot
  

• cleaning packages

  sudo apt-get autoremove
  sudo apt-get autoclean
  

  see list of all installed packages, sorted by size. If you see something big and don't use it - uninstall it

  dpkg-query -W --showformat='${Installed-Size} ${Package}\n' | sort -nr | less
  

• clean unused language files with translations (there are tons of them)

  sudo apt-get install localepurge
  

• check content of /var/tmp/

  du -sh /var/tmp/
  

• check also

  man deborphan
  

• Search for big files:

  find / -type f -size +1024k
  

  or

  find / -size +50000  -exec ls -lahg {} \;
  

• big installed packages

  dpigs
  

this is part of the package: debian-goodies

sudo apt-get autoremove

That can clean out a lot of guff (old kernels, etc) that have been replaced. You can do a similar thing in Synaptic (load it up and select the status button and then the Auto-removeable option).