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 File Usage Analyzer (AKA baobab; GNOME based), Filelight or kDirStat (KDE based), to see where the disk space is going visually (ncdu uses a TUI).

• Check if you have old kernels for deletion

   ls -lh /boot
  

• Cleaning packages

   sudo apt autoremove
   sudo apt 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 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 (part of the package: debian-goodies)

   dpigs
  

  or wajig sizes | tail -30.

• On systemd: Remove the oldest archived journal files until the disk space they use falls below the specified size

   sudo journalctl --vacuum-size 10M
  

• Limit Tracker disk usage.

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).

I just freed up almost 2 gigs by removing old kernels and header files:

use

uname -r

to check your current version, then

dpkg -l linux-image-* linux-headers-*

to see all the old kernels and header files, then

sudo apt-get remove linux-image-<XYZ> linux-headers-<XYZ>

the apt-get remove command supports wildcards, so you can do apt-get remove linux-image 3.0.* linux-headers-3.0.* for example, to get rid of many at once.

Make sure you don't kill remove current kernel of course! And maybe keep one or two old version, just in case... but not 10 or 20!

Do not forget to Empty Trash.

Or from command line:

rm -r ~/.local/share/Trash/info/ && rm -r ~/.local/share/Trash/files/  

Or just trash-empty using trash-cli package.

Uninstall large packages that you don't use

Some packages can be quite large. A convenient way of finding these is to use dpigs. It's not installed by default but can be found in debian-goodies (GitHub project)

sudo apt-get install debian-goodies

Then:

$ dpigs
419576 texlive-latex-extra-doc
204112 nvidia-319
175463 google-chrome-stable
141058 linux-image-extra-3.11.0-15-generic
113173 libreoffice-core
104822 valgrind
102322 qt4-doc
93337 blender
91105 texlive-pstricks-doc
90517 libboost1.53-dev

There are a few options:

-n, --lines=N
  Display the N largest packages on the system (default 10).
-s, --status=status-file
  Use status-file instead of the default dpkg status file.
-S, --source
  Display the largest source packages of binary packages installed
  on the system.
-H, --human-readable
  Display package sizes in human-readable format (like ls -lh or du -h)
-h, --help
  Display this message.

I really recommend you the program BleachBit, which cleans everything on Ubuntu. Install it using the following command:

sudo apt install bleachbit

Remove all old kernel versions automatically

Attention: If you've just upgraded the kernel, reboot before deleting the older versions!

Remember to check which kernel you are using type:

uname -r

Then as root:

sudo apt-get remove --purge $(dpkg -l 'linux-*' | sed '/^ii/!d;/'"$(uname -r | sed "s/\(.*\)-\([^0-9]\+\)/\1/")"'/d;s/^[^ ]* [^ ]* \([^ ]*\).*/\1/;/[0-9]/!d')

read that thread to be safe not to remove needed kernels with this command!

Check for large numbers of log files too:

sudo du -h /var/log

Or as rubo77 points out in commends, you could use the NCurses disk usage tool:

sudo ncdu /var/log

Cleaner script

This script will execute the biggest chunks on the console:

• Clean apt cache
• Remove config files left from uninstalled .deb packages (happens if you don't use the --purge switch with apt-get)
• Remove every kernel except the one you are using
• Empty the trashes of every user(including root)

create a file with this content and give it executable rights:

#!/bin/bash
# Adapted from 71529-ubucleaner.sh - http://www.opendesktop.org/CONTENT/content-files/71529-ubucleaner.sh

OLDCONF=$(dpkg -l|grep "^rc"|awk '{print $2}')
CURKERNEL=$(uname -r|sed 's/-*[a-z]//g'|sed 's/-386//g')
LINUXPKG="linux-(image|headers|ubuntu-modules|restricted-modules)"
METALINUXPKG="linux-(image|headers|restricted-modules)-(generic|i386|server|common|rt|xen)"
OLDKERNELS=$(dpkg -l|awk '{print $2}'|grep -E $LINUXPKG |grep -vE $METALINUXPKG|grep -v $CURKERNEL)
YELLOW="\033[1;33m"; RED="\033[0;31m"; ENDCOLOR="\033[0m"

if [ $USER != root ]; then
  echo -e $RED"Error: must be root! Exiting..."$ENDCOLOR
  exit 0
fi

echo -e $YELLOW"Cleaning apt ..."$ENDCOLOR
aptitude clean
apt-get autoremove
apt-get autoclean

echo -e $YELLOW"Those packages were uninstalled without --purge:"$ENDCOLOR
echo $OLDCONF
#apt-get purge "$OLDCONF"  # fixes the error in the original script
for PKGNAME in $OLDCONF ; do  # a better way to handle errors
  echo -e $YELLOW"Purge package $PKGNAME"
  apt-cache show "$PKGNAME"|grep Description: -A3
  apt-get -y purge "$PKGNAME"
done

echo -e $YELLOW"Removing old kernels..."$ENDCOLOR
echo current kernel you are using:
uname -a
aptitude purge $OLDKERNELS

echo -e $YELLOW"Emptying every trashes..."$ENDCOLOR
rm -rf /home/*/.local/share/Trash/*/** &> /dev/null
rm -rf /root/.local/share/Trash/*/** &> /dev/null

echo -e $YELLOW"Script Finished!"$ENDCOLOR

Adapted from 71529-ubucleaner.sh

Find large folders and packages

Some tools that will help you find large folders and packages:

sudo apt install ncdu debian-goodies deborphan
sudo ncdu -xr /  # lists all folders by size on the console (like the gui `baobab`)
dpigs -H         # shows large packages that you don't use
man deborphan    # finds packages that have no packages depending on them
deborphan --guess-all --libdevel | xargs apt-get -s purge

Localepurge

Use localepurge to uninstall unused languages in your system:

sudo apt install localepurge

Remove old snapd images

LANG=c snap list --all | awk '/disabled/{print $1, $3}' | \
    while read snapname revision; do \
        snap remove "$snapname" --revision="$revision"; \
    done

More here.

Bleachbit

A great graphical GUI tool to fit all needs:

sudo apt install bleachbit

More here.

Remove manpages and documentation

Additionally you can remove manpages and documentation as described in the Ubuntu Wiki:

Create a file /etc/dpkg/dpkg.cfg.d/01_nodoc which specifies the desired filters. Example:

path-exclude /usr/share/doc/*
# if we need to keep copyright files for legal reasons:
# path-include /usr/share/doc/*/copyright
path-exclude /usr/share/man/*
path-exclude /usr/share/groff/*
path-exclude /usr/share/info/*
# lintian stuff is small, but really unnecessary
path-exclude /usr/share/lintian/*
path-exclude /usr/share/linda/*

Remove the same set of files and directories in the project-config's postinst. Example:

echo "Removing documentation..."
# if we need to keep copyright files for legal reasons:
# find /usr/share/doc -depth -type f ! -name copyright | xargs rm || true
# else:
find /usr/share/doc -depth -type f | xargs rm || true
rm -rf /usr/share/man/* /usr/share/groff/* /usr/share/info/* /usr/share/lintian/* /usr/share/linda/* /var/cache/man/*