There are a good number of sites out there that will tell you how to keep your iTunes library synced between two machines. For my own reference and in case it's helpful to you, here's how I do this using a firewire drive. I try to only update one machine at a time. Usually that's my desktop, unless I have a gig coming up, in which case I move everything over to my laptop where the dj software lives. I use rsync; the only really tricky thing is getting the trailing slashes right. This seems to work.
On the most recently updated box, do
rsync -aveP --dry-run ~/Music/
/Volumes/2010Backups/Music/
Then delete the "--dry-run" and actually make it go. Eject and uplug the drive, and plug it into the on the less recently updated box. Then do this
rsync -aveP /Volumes/2010Backups/Music/ ~/Music/
