Clochette is a very easy to use application that allows you to rename the files (on the file system) that correspond to selected tracks in iTunes.. in a fast and smart way.
Note: Make sure that you have disabled the Keep iTunes Music folder organized option in iTunes preferences (Preferences > Advanced). Otherwise, iTunes will rename your files again.. its proper way.
In the iTunes main window, select the tracks that correspond to the files you want to rename .
In the Applescript menu (still in iTunes), click the menu item labelled Clochette File Renamer.