Something we do is ask the actors and crew on the show tofill out a survey at the closing performance.  It lets us know what we're doing well and what needs improvements.  And it does look good on grant evaluations when you can quantify that the volunteers valued their experience on a show.

Another measure would be to do audience surveys.  Don't do them at every show though because then people will just become annoyed with them.  We do an extensive audience study every three or four years and use those statistics heavily in grant applications.  In our last survey one very useful (from grant writing stand point) question we asked as to state our mission and then ask them how they felt the organization was doing in achieving that mission.