There is no one solution for this. It's part marketing, part luck, and a lot of effort on your own behalf. Sign up for similar communities, put your site in your signature, and become a part of those communities. When I first started my site, that's how I got the majority of my memberbase.

Don't just plug your site, take the time and actually contribute, be a part of that community, and eventually members will start to use your own site as well. You can't just slap your site in your signature and post nonsense once a month and expect to gain new members. As you build online relationships, your own site will start to grow in kind.

Other than that, SEO, solid content and things that make your site unique and worth registering at are basically the keys. Ask yourself why you sign up for forums, and what you want to see. Then craft your site around those things and put in the work to make it look good, work well and be a place that YOU would like to visit daily. As always, the biggest key to it is patience.

Edit: Also, the paid modification forum here, for me, has been great in terms of making my ideas happen and generating traffic as a result of things that I thought would be great additions to my site but didn't have the time/skill to code them myself. I've paid about ~$500 in custom mods to coders here in the last two years, and it's been well worth it.