I wouldn't begin SEO unless you really know the background of it all and white hat search engine optimization. Last thing you want to do is causing negative noise for your forum. I would recommend exactly what you said for us not to recommend, Forum Promotion. Use our services, use other forum services, visit the exchange and complete post exchanges etc.
You will need to get quality posted content on your site too, this is driven (or should be) from your member base if established and should be fairly easy to obtain, take a look around other forums and see what topics they have from months back when they started. Keep in touch with news stories, cook up some good debate discussions.
If you have your forum on your own hosted plan, I would strongly recommend a wordpress CMS main site and move your forum to a /forum sub-folder. This can drive huge amounts of traffic to your forum, is very easy to update, skin and manage & it is all free.
Lastly, I will suggest that you take on-board directories that you can submit to, one being a
forum directory that we have here at FP.
Best of luck!
