What is a membership site?
A membership site provides information or services to those visitors who signup and pay to become a member. Some membership sites bill their members on a monthly basis, while other have a one time fee. Once someone signs up as a member, depending on the owner's configuration, they are automatically billed each month. Those who sign up to become members have access to member's only information, benefits or discounts provided by the site owner on a regular basis.

Content Based
If you can produce information about a certain subject there are many ways you can go about launching a membership site and making money from that knowledge. You can set up a blog like website where users can signup, for a fee, to access advanced information updates from your website.

You can also set up an informative website where members have a clear way of participating with you as well as the other members on the chosen subject. This builds the sense of community which increases the number of renewals.

You don't have to be a permanent presence either. Simply let your members know that you will be available X amount of times monthly or weekly from say 12pm to 2 pm to answer questions... you get the idea. Another option that won't require a set time commitment of you is to create a forum where you can manage your time to help members as you can.

If you are not an expert but want to learn about a specific subject, acquire a certain skill or even study how to improve your golf skills, you can become a spoke person for people who have the same interest. Contact a few experts in your field of interest, record presentation and make it available to your members.

Member's Only Benefits
Offer more information of your products to members at a reduced rate. For instance, if you have a fellow business owner who sells a product your members would find useful, see if you can negotiate a special deal for your members only. Most business owners will accept since there's no investment for them.

Do you offer a service or know someone who does? Again offer your members a discount created specially for them.

Affiliate Offers
Affiliate marketing is another way to generate sales from membership sites. If you promote products as an affiliate, be sure to include those promotions in emails and posts to your members. As with the previous money-making option, you might even be able to set up a discount just for being a member or give them an added bonus of your own when they purchase through your affiliate url.

If decide to add affiliate links to your website, make sure the information you're offering is related to your main subject and important to its members.

Once you have decided which strategy you want to implement, you need the right tools. There is a lot of information available about how to start a membership site all the way to premier hosted solutions. Sites like iLearn Online Marketing or My Membership Empire have vast training courses, membership site blueprints and materials.