I ask because Ubuntu Tweak's Janitor is frozen due to the sheer number of packages in the cache that accumulated during the upgrade from 12.10 to 13.04.
While I could use the terminal to perform this, I'd prefer the GUI.
In Synaptic , go to Settings → Preferences → File tab. Choose only delete packages that are no longer available and you're done.
You can also use Ubuntu Tweak, but it takes much longer to do the job.
An update - I've been using BleachBit for some time and the Superuser version lets delete the apt-cache and a lot more.