There are plenty of effective ways to sync your iTunes libraries, like using MediaRover or, my personal favourite, Dropbox, but your Mac already comes with the tools you need to solve the problem: rsync and Automator.
MacRumors has a great guide on this process. Essentially, you use the great command line tool rsync to synchronise two iTunes libraries. That's a manual process, however, but one that can be automated with Automater — another free tool on your Mac.
The process is fairly simple. First you need a working rsync command, which will look something like this:
rsync -av —force —delete —size-only SOURCE_ITUNES_LIBRARY DESTINATION_ITUNES_LIBRARY
Then you need to set up Automator to do the work for for you. You'll need it to connect to your other computer as a shared volume, run the script, then eject the shared volume. To get the full rundown, check out the guide over at MacRumors:Guides.
For more great Automator tips, check out Whitson's great Automation guide for Mac.
Keeping two iTunes libraries in sync [MacRumors:Guides - Thanks for the tip, Kentaro!]