Here's what I suggest for all new forum admins especially a good niche one like yours:
1) Of course start out your initial advertising campaign. As many forums as you can find and more importantly SE submissions and DMOZ. It's important to get this out of the way early because this is what will take the longest to produce any results in terms of actually getting listed.
But don't focus too much energy on this yet. In fact don't even spend money on it (unless you just can't resist but that's your call). I'll explain why in #3.
2) Get those initial members first. Not only will they provide the posts you need to keep passers-by interested and convert them to signups but they will also bring in new memebrs through word of mouth. They are also usually the most loyal especially since they usually become moderators later on

.
3) Once you have some content then you can step up your advertising campaign. PPC advanced SEO techniques, link exchanges, the works. Why did I mention this all in this particular order? Simple, you don't want to waste time and effort on promotion and SEO to attract members to an empty forum! Your signup rate and ROI will be a lot higher if your visitors actually have something to read when they arrive.
Find other related sites that aren't forums and pay for ad space.
The above formula (I like to call it my "forumula"

) has worked wonders on nearly every forum I've ever created. I own a lot of forums that I NEVER mention here on SPF just because of their level of success and niche, I don't want clones. But an example of one that I can talk about is in my sig. EnvoyForums
is less than a month old and is closing in on 400 posts with an average of 1 to 3 new members per day and rising.
With forums it's an easy cycle..
content -> traffic -> more content -> more traffic


