First, take a good look at usage patterns on your site. What % is from repeat users? How often do repeat users return, on average? What % is from search referrals? What % is from referrals from other sites? What search terms are popular? Just looking at that data and thinking about the implications could be instructive.
I admire your stance on ads, but you might reconsider your decision not to show them to logged in users at all. You are providing a service by hosting the forum, and its not unreasonable to have a small amount of advertising to your regular users to help defray your costs.
This is important, because I'm guessing a significant amount of your page views are for regular users, and because they are regular users, they represent a targeted audience for advertising. Because they are a targeted audience in what sounds like a commercially viable niche, you should be able to get pretty good CPMs, and therefore, be able to realize a big boost in revenue without having to show them a lot of ads.
I'd start by putting a decent sized google ad (preferably one of the formats that supports image and video ads) "above the fold," but only show it intermittently, and not more than a few times per visit. Leave it there for a while and see if Google starts to identify your regular visitors and targets them differently than it targets passers-by (set up an ad channel to help with tracking).
You could then try moving beyond that by joining an appropriate display ad network, and doing a little work to sell ads yourself. These would probably be useful for monetizing your other users as well, but you'd probably command a higher CPM for your registered, regular users.
Your eCPM for AdSense doesn't sound bad, but you cant really be sure based just on the info people are giving you here. You need to try and benchmark your eCPM against sites that are similar to yours. You might be able to sign up for an AdWords account and see what it would cost you to buy ads for the keywords of the ads that tend to show up on your site.
Unless you've put a lot of effort into optimizing your ads, chances are that a little bit of work could get you a 50-100% boost in your revenue. Experiment with different ad formats, placements, and color schemes. As someone mentioned, just introducing some variation in color scheme (which Google makes easy to do) can make a difference.
Another thing you can do is have a prominent "landing site" that is displayed when you detect that a user has arrived via a search engine referral. The site can feature a larger ad than you'd usually show, you might also include a link-unit, and links to other content on your site that might be relevant to the search terms they used on their way to your site.
The last thing to consider is how you can make the best posts on your forum site easier for casual readers to consume, for search engines to find, and for Google to target. One approach, you could spend a little time maintaining a blog of the best posts on your site. Make sure they are tagged and categorized appropriately (pay attention to the popular search terms for people arriving at your site when choosing tags and categories). A little editing to provide the appropriate context, and make them easier for someone to read without reading the whole thread they were pulled from is valuable too. You'll end up with pages that aggregate a bunch of content on the same subject. This will be easier for search engines to index, easier for people to read when they arrive from a search engine or another site, provide a nice target for people to link to (which can help with your search engine rank), and also provide a crisper context for Google's ad targeting.
You could try and recruit a few of your most devoted (and best suited) users to help with this, and you could pay them a share of revenue from the blog pages.
You could also go a different direction, and start a wiki with similar content drawn from the forums, and then encourage your users to expand it and keep it up to date.