Think like a customer. You want traffic that is looking to spend money and are looking to purchase products . mancar487 suggested in in his Cb 5-min megathread about targeting products for niches that keep people up at night (something they will instantly buy if they think it will help). This would include self-help, remedies, treatment products, etc.... (people trust other people's reviews when they believe they aren't directly selling the product - but it needs to be more then just a random "spammy" forum post)
Traffic on forums usually are not looking to spend money (depending on the niche), most are looking for free information.
So again - target buying customers. This applies to many different IM techniques. PPC, email, video, etc
Think like a customer..Great thinking.