Just highlight the tracks you want in your iTunes library and drag them to the open folder representing the drive.
This will put a copy of each on the drive, and does not change anything in your library.
If you normally run iTunes in full-screen mode, make it smaller so that you can do the drag.
Try it first with just 2 or 3 files so you can see how it works.
but I actually know how to do it manually. ...and that's not really solving my issue at all. As I add files to the playlist, for example, I have to remember what's new, what's already been copied to the device, etc...
If this device were attached via USB cable, I could just use something like Missing Sync's GoGadget software but I'm having a hard time finding something that will sync the data to a mounted drive rather than a USB connection.