OK, being honest, I think you are taking the wrong approach here. Think of your demographics. Who is actually using this site? The domain says it all. Webmasters.
First of all, webmasters are, in my opinion, more susceptible to ad blindness. Therefore contextual advertising is less effective. Also, there are not many products to advertise to established webmasters. We tend to listen to our peers more than banners and contextual ads. I'm not saying it doesn't work, just that it will be less effective.
Think who would want to advertise here? And in what format?
Members would like to advertise here. Text link advertising. I know you use TLA, but why not cut out the middleman? (You thought me that from our discussion on domain parking).
Now, lets start brainstorming. What do other webmaster forums advertise? I know v7n advertise their directory. Don't you have one or two of them to cross promote?
Again, it's your forum and you can do what you like, that's just my 2 cents.