A Macbook Air is my weapon of choice- however after all of my apps are installed & my archived design files- I don't have much room on my hard drive. So- I figured out this little trick to store tons of albums on an external drive, thus freeing up some space on my work computer:
1) Copy all music (iTunes folder) to the external.
2) Quit iTunes & then hold the Option key on a Mac (Shift key on Windows) and launch iTunes. Hold down that key until you're asked to Choose iTunes Library.
3) Point it to the external iTunes library.
4) You may need to "add files to library" again- but you get the gist.
iTunes will remember this setting every time you launch it w/ the external hooked up & voila- you can delete the MP3 files from your computer's drive (once you're sure you've backed up everything correctly).