Why don't you just hire a
SEO specialist to help you promote your forum?
Well, that can do a difference but if you can work the promotion, marketing and link building of the site then that will be great!
Aside from Forum posting with the signature attached (please read our Forum Rules on Signature), you can also create Press Release or Ads and submit it to either free or paid submission sites.
Further, you can also write articles about your products and target keywords through anchor texts and do article submissions.
Plus, you can also join and register with social bookmarking/networking sites. That's one of the fastest way to drive traffic to your sites. The more traffics you earn, the more chance of getting sales.
Good luck!
