It's funny. Back in March I went through all of this worry about how to grow my Web site. Getting people to post was like pulling teeth. People would see that I didn't have a lot of members, and they would flee. Bad cycle.
Now, I have almost 1800 members, and when a new member doesn't get his activation email within minutes, he or she will send me a handful of panicked emails. If I go away for a few days, there will be 100 new members when I get back. Frankly, I'd gotten used to this.
Now I'm launching a new Web forum, and I have 12 members, and getting new members is like pulling teeth! Again! So I'm thinking about what I did with my previous site. I have a few thoughts.
First, people are attracted to a crowd. So the more members you have, the more members you'll get. It's partly from activity -- lots of posts -- but also partly psychological. Seeing a forum with 25 members is kind of sad, it makes people want to go elsewhere. I just removed the member count on my new site for that reason. On my other, older/successful site, I think by the time I hit 1000 members, the momentum was in my favor. I'll get my second 1000 members in half the time it took to get the first 1000.
Second, your members are your advertisers. They're going to tell their friends about your site. Again, this is sort of a momentum thing -- once you have a lot of members talking about your site, you'll get a lot of new members. In the beginning, you have to artifically stimulate this activity with things like "email this topic" links. V7 has this under the "thread tools" link. phpBB can do it with an optional add-on. My new site currently doesn't have this option, so I know I need to add it soon.
Third, a good design really does help. Once I fully customized my home page (and later themed the forum itself), it was a little easier to sell people on the concept. My new site could take some lessons from my old site, in that regard.
Fourth, yes, you actually have to go through the grind of submitting your site to search engines, directories, and so on. I used to use automated tools, but they were worthless. With my successful site, I spent a 3-day weekend visiting directory after directory, adding my URL to any that were free. Now for my new site, I have to do this all over again. This is partly important to give you page rank, although to be honest, Google is wise to this stuff now, and doesn't really count directories much. But also, some of the directories will actually drive a few visitors your way. My logs show that I get about 1 to 5 visitors/day from all those directories combined. That may seem to be not at all worth the effort, but consider this: I worked my butt off for 3 days straight 9 months ago, and every day since, those directories have sent me a few visitors without any extra effort on my part.
Fifth, signatures cannot be underestimated. You must must must find a dozen forums that are close to yours (maybe not direct competitors, but complementary in some way), set up an account and a good linky signature, and start posting. What's important to note is that it doesn't work to post in unrelated places. For example, here on V7, my signature has links to non-Webmaster stuff like Narnia & writing. I won't get much mileage out of this post, as I can't expect Webmasters to be obsessively attracted to things outside their interests. However, I also post at some writer's forums, and after each post there -- assuming it's good -- I can expect to see 5 to 20 visitors to my site.
As a side note, you have to be careful about the forums, size-wise. In my bookmarks, next to each forum I've added a ranking. The ranking is a rough mashup of both how many members the forum has, and how many visitors they send my way when I post something good. I direct my time to the ones with the biggest bang for the buck. That's a little harsh, because discriminating against small sites is EXACTLY the problem your small site is experiencing, but you need to concentrate your efforts on the places that have interested people.
Some smaller sites will do trades -- you post 50 times to theirs, they'll post 50 times to yours. I consider that an excellent deal.
Also, there is one guy on one of these Webmaster sites that does forum posting for a living. You buy 100 posts from him at a time. He's willing to give himself multiple-personality-disorder for a day, and post from 3 or 4 different accounts. I also consider his service to be worth it. It can help you to get over the initial hump (I think until you get about 250 members, you're humped), and it can bulk up your forum. I've never used him, but if I track him down again, I just might.
Hmm. What else did I do? I did Google Adwords. Still do. But Google is starting to charge more, so I use them less. I use the Yahoo/Overture ads more now. There, I can still get decent placement for 10 or 20 cents in most cases. Search engine ads are very important for a new forum, especially if you're sandboxxed by Google for a few months.
Lastly, you HAVE to have a value proposition. I forgot about this. With my new Narnia site, I wanted to get people talking about the actors of the new Narnia movie. So I used their names as keywords to trigger my ads. I got 200 visitors in the first weekend from that. 200. Guess how many signed up? 2. I wasted tons of money. Here's what I did wrong: I offered nothing of value for them. I wanted them to come to my site and start conversations. But there were none that already existed. So they saw me advertising "discuss actresses Georgie & Tilda" but when they got to my site, there was no Georgie or Tilda discussion happening. They would have to start it. And apparently, they all bailed instead.
So I killed those ads. Now I'm getting more clear on what I do offer already, and I'm only advertising that.
Anyway, there's more, but this is far too long already. I hope you can learn from my mistakes. Basically, know your target audience, talk to your target audience, and give something worthwhile to them. Good luck. To both of us.