I am having the same issues with my new forums,
I get on average 4 clicks a day with a $1 limit on yahoo ads (googles ads wouldnt go past to the payment section i tried loads but it wasnt having it), So i have invested in this for 1 month to see how it goes.
I also put links in my forum sigs, and since i use similar forums myself, your basically posting your link to your target market every post you make! Its a good way if your a common forum user.
Check my forums in my sig, I am offering a free e-book of $19 value to new members who keep active. And as weblord said, forum gold, I installed an app for it on my forum but it wasnt working, when i find a working one, i will allow users to purchase my e-books that i sell using forum gold, and possibly sell other things, I am new to this kind of market, so I am still researching.
Good luck in marketing your site, if you need help join my forums and ill join yours, we can try to help each other get some traffic!
NVD