One of the most lucrative ways to monetize is to create a membership site, where people pay you a regular fee (usually monthly) for access to targeted content and/or services.
Memberships can be structured in a variety of ways.
The most basic type of membership is quite simple - you just email your members a targeted report or other content once a month.
There are also much more complex memberships that provide multiple products and services. For example, the Authority Site Center's membership includes a variety of elements - training, a forum, publications, coaching, site hosting, videos, and more. Each of these components must be created, managed, and delivered - and also include support or troubleshooting when necessary.
Memberships can also be anything in between these two extremes.
Be sure to read the Membership Sites thread in the ASC forum for an extensive discussion on types of memberships, advantages / disadvantages, and ideas for each.