Set the Plays ("Play Count" before iTunes v9.2) of the selected tracks to a user-entered number (they will all be set to the same number). If this number is 0 then the Last Played date of each track will be erased. (iTunes has the ability to "Reset Plays" to 0 from a track's contextual menu.)
Also see New Last Played Date and Add or Subtract Play Count.
Applet assists with swapping, copying, appending data between track tags
Append user-entered text to beginning or ending of selected tag of each selected track
Edit basic tags for a selection of mixed-media tracks
Exports and then re-imports selected tracks' artwork
Remove iTunes tracks disassociated from files
Drag-and-drop to convert audio files with iTunes