Don't 'market' it by email. No matter how you call it, it's SPAM. I get very annoyed when people "invite" me to see all kids of crap. Believe me it won't work.
Work on the design: I know many claim it's not important, an excellent design will get you noticed. if the forum is "under" a main site, make sure the design looks the same, so that you don't mess up the "main brand". My latest forum (gofreelancing.info) got some initial signups after I showed it into some forum administrators' communities, becase the design looks so interesting, they loved it. And joined. a good design can get your community showcased in sites that list "nice designs", so you're getting some free traffic from that too.
Organize the information. Don't start with 2000 categories, it's useless. I have the following forum organization in most my forums:
Community (main category)
--- say hello (forum where they can say something about themselves and present themselves to the community. Many new people are "afraid" to enter the community, we can solve their shyness this way
--- offtopic (anything that;'s not your topic: music, favourite TV shows etc.) .. there's a debate in the "industry", if we should have this or not, the good thing is that sometimes people can get less shy and chat there and then get the courage to enter the rest of the topics.
--- news and feedback - you have the rules here, their feedback, announcements etc.
"YOUR TOPIC" forums (main category)
-- general "YOUR TOPIC" discussion - start with this category and then branch out from it. Or have 3-4 subforums for the niche and, when the forums get active and you have more stuff, move topics to new categories.
You can take a look at the forums I've presented before and see how I've arranged my content.
Start 30-40 initial topics. You won't start promoting till you do this. Start with questions that make people want to reply to: ex: "how hard was for you to start freelancing", "did family support you", "do you still recall your firs freelancing job" ... gave examples in my niche.
Go to forum admin communities (Theadminzone.com for instance) and discuss your new forums and get ideas for improvement. There you can also get some post exchanges or paid forum posting. They might ge frowned upon by some, but it can kickstart your community. If you're smart and move OK, in 2-3 weeks you might move OK with the forums.
Announce your subscribers from the main site *if you have them, about the new community.
start an advertising campaign online (adwords, buysellads etc.)
Comment in similar topic blogs and put the link. Comment in similar forums and have a link in the signature.
Keep on adding content and work on the forums.
It's not too hard to make a forum work, but it's not too easy either.