Traffic has three elements, whether it's a Forum, Blog or plain ol website. You need (1) Source, (2) Mechanism, (3) Bait.
This is where you have to leverage on places that already have traffic. You latch on to these sites for traffic, which is either bought or traded. You might consider also running joint teleseminars with similar experts to drive traffic to an opt-in list where you continue to feed them valuable information.
There are a whole bunch of these. Blog contributions and trackbacks, pinging, search engine optimization, link exchange, article directory submission, press releases on PRWeb.com, RSS submissions, social networking, social bookmarking, pay-per-click advertising, etc.
No use having traffic pass through once, right? Have them come over again and again! Articles and writing are one, but have you ever considered Podcasting? iTunes (though I hate the format) has over a million people searching for content on Quicktime. You could also do up a quick video, convert it to flash, upload it on YouTube. AskTheNinja.com is one example of a hilarious, viral site among others. Superior content wins out.