From time to time, you could run command:
sudo apt-get cleanto remove all those downloaded packages.
If you only want to remove the useless old packages, you can use command:
sudo apt-get autoclean
You can also do it with the graphic tool Synaptic Package Manager. Click its menu Settings|Preferences. In the pop-up Preferences dialog, select Files tab. Under the Temporary Files section, choose what you want to delete and click the Delete Cached Package Files button.