~/Music/iTunes/iTunes Library
~/Music/iTunes/iTunes Music Library.xml
Move your music files to their new location. Figure out what the new location of your files is in file:// URI notation. The location will be something akin to this:
file://localhost/Volumes/DRIVE/PATHfor another drivefile://localhost/Volumes/SERVER_SHARE/PATHfor files on a mounted network share
Delete the ~/Music/iTunes/iTunes Library file and move the ~/Music/iTunes/iTunes Music Library.xml file to the Desktop. Then open the iTunes Music Library.xml file using a text editor. Run a search-and-replace in order to substitute the original file locations with the new locations. For example:
file://localhost/Users/username/Music/iTunes/iTunes%20Music/ becomes file://localhost/Volumes/SERVER_SHARE/PATH/
Now launch iTunes; it should show up with an empty library. Choose File » Library » Import Playlist, and select your newly-modified iTunes Music Library.xml file. Wait for iTunes to finish importing (this might take a while, depending on the size of your library).
Enjoy! One note of caution: you may lose your Podcast settings this way; you will need to resubscribe to podcasts afterwards. If you make a mistake, or something fails, you should be able to recover the previous iTunes state using the back up you created before you started.
This hint should be future-proof, because it's based on an official Apple Knowledge Base article.

