How about charging for membership? Perhaps some kind of tiered system depending on how much you pay?
(Sorry, couldn't resist. Personally i have no issues with selling out the Guests with google ad words.)
I endorse this idea.
I also endorse Matts Google ads for Guests idea.
Is the referal link still added to links posted? I'm sure it was then got removed, why not put it back for all those of us who forget to use it.
Probly worth bookmarking it too.
(added to my list of things to forget to do later).
As for the AdSense / hosting costs...
How much data transfer do we have? - I remember it being something huge like 500GB a month, and we'd have to have quite a lot more visits to reach that (I'd be surprised if we're even near 10% of that at the moment.)
I think pushing the affiliate stuff is more likely to boost income than AdSense - how many people actually follow AdSense vs affiliate links?