Twice in my career I have run membership organizations. In each case building membership was important to our work. And I learned a lot about that from others who had been building membership successfully for years.
These are five of the better tactics for building and retaining a strong and lasting membership in my experience:

Personal Selling: Everyone at the organization can be trained to invite people to become members. They have to know the benefits and accurately explain them. I’ll never forget receiving a business card from a colleague that had a brief membership form printed on the back. He considered every card he handed out a chance to get a member. Ask people to join every time you give a talk to a civic organization and track the percentage of your audience that joins.
Free Memberships: Give a membership to anyone who attends one of your special events, conferences, seminars or webinars. If you charge non-members more (as you should), make the amount equal to a year of membership so that they see your services for a year and hopefully stay part of the family.
Invite People to Join at Entry: If you have a gate fee or entry fee to property, start with “Members get in free year-round. Would you like to join today and get in free now and for the coming year?”

Give Away a Trial Membership: Many organizations have found that asking folks to sign up that first time is the challenge. I once added a fee for use of picnic grounds at a nature center and to soften the blow of the change, we gave everyone who came out to picnic a free membership for that first year of change. It built goodwill and many later renewed on their own.
Offer Auto Renewal: If you give people the option of renewing automatically, many will take it. When that annual reminder comes, some people simply miss it or don’t get around to sending a check. Plan it so they can stay in your network as long as they wish without extra effort.
Membership may or may not be a large revenue source to support other activities. Often you must spend as much as you take in to provide benefits to members. However, they are also giving you money in annual campaigns, capital campaigns and as bequests in their estates. They are selling your organization to friends. They are your advocates when political problems arise. And some of them will emerge as major donors of long-term importance or volunteers.
They are your warm market, your family. Recognize them by name when you write about programs, recognize their gifts publicly on the Internet, in physical locations and with permanent markers when it makes sense for the size of their gifts.
There are many more ways to grow your membership but these five key approaches can get you started.
-Tim Merriman