# 1. Clean the APT Cache (And Do It Regularly)
sudo du -sh /var/cache/apt/archives
sudo apt-get clean
# 2. Remove Old Kernels (If No Longer Required)
sudo apt-get autoremove --purge
# 3. Uninstall Apps & Games You Never Use
sudo apt-get remove package-name1 package-name2 
sudo apt-get autoremove
# 4. Use A System Cleaner like BleachBit
sudo apt-get install bleachbit