With any sort of survey, the best way to ensure compliance without
annoying your attendees is to educate them as to the hows and whys
of the process and how this will benefit them as museum attendees or
members.

A bald "You must fill this out or we won't let you in" won't get
many responses and the ones it does generate will be pretty angry
(as well as possibly untrue).  One of the major electronics
retailers in North America has a apolicy of asking for your name and
address information on every purchase --including cash purchases.  I
no longer frequent that chain and do not even mention them by name,
so as to limit any sort of business that I might unkowingly generate
for them.  While I doubt a museum attendee would react so harshly to
a poorly-phrased survey request, it is always better to err on the
side of caution and courtesy.

As practical hints for ensuring a successful polling effort, I'll
offer the following:

Post signs and include notes in your literature that polling will be
taking place.  Explain the reasons why it is necessary and how it
will benefit the attendee.  Note that all polling is voluntary.

If at all feasible, do your polling through the mail.  Include some
sort of incentive (a free day pass or a mug or somesuch) for the
attendee to complete and return the form to you.

If you do on-site polling, train your pollsters to be courteous and
to be able to respond to questions your attendees might have about
the poll.  If an attendee declines to participate in the poll, take
the "no" with good grace and finality and let the attendee get on
about their business.

When you complete your polling, let the members and attendees know
about it and explain the results of the poll.  You'd be surprised
how many people are genuinely curious to see how their opinions
match up with other members of the polling populace. This also gives
a sense of closure to each polling effort and avoids statements by
your attendees like "didn't we go through this last
month/quarter/year?  Whatever became of those results?"

Finally, for those museums who have a scientific or statistical
bent, set up your polling place as an on-going intereactive exhibit.
Do make sure that all of the information about the hows and whys of
the polling and the use the data will be put to are posted clearly
for all to read.

I hope this helps.

Rich Johnson
Director of Marketing
Cotton Expressions, Ltd.
http://www.cottonexpress.com
Makers of science and science humor t-shirts