The most important thing is to offer relevant, regularly updated content.
That provides an incentive to join.
Also, you need traffic - focus on improving search engine rankings, link exchanges are a good way to do this.
Also, linking to your site on other sites you are a member of, e.g. setting it as your homepage on this forum, will increase traffic.
There are many more ways to get more members, but relevant content, a decent site layout, and a core of active members are the only things that will persuade members to register and stay active.
