The constant search for donors is what prospect researching is all about. A great article about membership's came to my attention recently through www.nonprofitbuzz.org, which by the way is a great source of news and tips for people looking for donors. This particular piece explains the pro's of being and having memberships, some of which are:

1.reaching your target audience
2.getting discounts/benefits with your memberships
3.knowing that you yourself are contributing to a cause that you feel passionate about

Sometimes you have to pay before you can receive. Here is the link to the article on membership: nonprofitbuzz.org/membership.htm