This is kind of related to the donation jar discussion...but the best thing we ever did was make membership categories above just basic. Our basic individual membership is $20 (don't sell yourself short here either...the cinema these days is roughly half of that!) and then we have a family membership at $35 and then other categories like Friend - $50, Supporting - $100, Patron - $250...right on up to $5,000+
We don't give any coffee mugs or mouse pads or anything special to the upper levels. They just get their name in a different category in the newsletter list of new and renewed members. (we do invite our $100 and up members to a special Christmas social)
We discovered that people will almost never fill in a membership form as a basic member ($20) and fill in the "Donation" line with an extra $80. What they will do however is sign up at the $100 level because it is there and there a little box to check next to it. It has worked like a charm for us.
If you can come up with incentives all the better I guess but we have not found a need. Plus some people are turned off by the "free" gift you are sending to them at upper levels when we all know that it cost something. (money which many people would rather see put to good use in the organization)
Another point while I am on the subject...you must have an aggressive membership retention program (i.e. renewal letters, thank you for joining letters etc.) If they don't respond to a membership renewal letter the first time, try it again in a few months. If they don't respond after a year drop them off your list and for heaven's sake don't give away newsletters to people that aren't in relatively good standing. They'll never renew if there getting the newsletter for free anyway.
Donation boxes are great but you must tell visitors that donations are greatly appreciated. Don't expect too many people to give just because it's there. We all have to be unafraid to ask. Also push membership at that time.
Paul Wolter
Sauk County Historical Society