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 Add or Subtract Play Count.
Perhaps Also of Interest...
downloaded 9m 58s ago
Apply selected CD's CD Text to its disc and track tags
downloaded 22m 18s ago
Move selected playlists to a new or existing folder
downloaded 32m 38s ago
Copy multiple playlists from iTunes to a mounted iPod.
downloaded 37m 48s ago
Perform tasks on a batch of playlists; rename, delete, duplicate, merge, more
downloaded 1h 10m 28s ago
Export files of tracks in two or more playlists to single folder
downloaded 1h 14m 57s ago
Rip CD tracks with different encoders in the same session