Ads are cheap to buy and cost nothing to produce because it's all done in email. Make sure you advertise on a moderated list where the moderator keeps the discussions on-topic. Ultimately, whether you advertise with banners on sites or on lists, make sure you give your target group a darn good reason to come to your website. You must must must consider offline promotion of your website as integral to your plan to attract traffic to your site. Maybe your content or attraction is so good it gets talked about and pointed to in the trade press.
Nothing builds traffic like a third-party editorial endorsement.
Print or broadcast ads are a big help too, if you have the budget for it. Remember, you can do a barter deal with a print publication the same way you can with another website. Think about having an event on your site to draw in visitors. Even better, design a campaign with weekly or monthly events. Whatever it is, it has to be snazzy enough to cause them to write down the url, go to their computer, fire up their browser, and then go to your site.