Automatically move old music tracks on iTunes to external HDD
Steps to perform this:


Navigate to the location you want to copy the files to and leave the
window open in the Finder.
Create a smart playlist in iTunes by Option-Clicking the new
playlist button in the bottom left of the main window.
Configure the playlist with the appropriate criteria, such as:
{Last Played} {is before} {1/1/2008} or whatever you consider
"Old."  Click Ok 
Select Edit, Select All and drag the items to the folder you opened in step 1.  Wait for the copy to finish and you'll have all your old music in that folder.
To automate the process, use Automator and configure a new workflow similar to the following image:
Rig up the workflow you just created to run.  Put it in the dock, use Crontab or run it at login.