About a year ago, our director of visitor services placed notebooks labeled
"Talk to Us" throughout the museum.  While many of the comments were one-
liners at first, once visitors became accustomed to seeing the books, the
comments became more thoughtful.  That's not to say that we were (or are)
overwhelmed with visitor comments but visitors, and especially frequent
visitors,
know that we want to hear from them.  Also, just having staff on the floor
invites comments, as was the case this past Friday.  On several occasions,
visitors made it a point to say how much they had enjoyed their visit and why,
as well as conveying constructive criticisms.
You mentioned using a variety of formats to reach your visitors; whatever the
methods, it seems to me that you need to use one or two formats on a regular
basis--and above all, it's vital to have an element of personal communication.
Good luck!
Anne Arenstein
Director, Public Programs
Museum of Natural History and Science
Museum Center of Cincinnati
1-800-733-2077, ext.7067