Former ad/pr guy here. Don't know a whole lot about Internet advertising specifically, but I suspect any potential sponsor is going to want to know numbers like # of members, how often they visit, how long they browse the forum, % of visitors who are in business or who would otherwise buy their products/services, etc. You want to show them you have as large an audience as possible, as affluent an audience as possible, with lots of sales prospects. I get the feeling the site is fairly new, so if you can show the (I suspect) explosive growth in # of members since site inception, that would probably help. I'm sure I'm not telling you anything you don't know already, but throwing it out there, anyway.
Anything you can do to get more eyes on the site, the better – including flame wars, etc. More reader engagement (polls, questionaires, promo giveaways, buy/sell/trade, etc) is always good.
I bet a Google search of "how to sell internet advertising" or something like that would yield a lot of good info...