My website has 1 million impressions and 200,000 visitors a month - how do I make money from it?

I've been running a very niche website for medical travel for 8 years. Its receives 1 million impressions per month and around 200 thousand visitors per month. I've tried all sorts of advertising models but they haven't worked very well. It is an international website, with around 50% from the US, 35% from Europe and 15% from the rest of the world - its only available in English at the moment. The website is number 1 for several niche search words on Google and Yahoo and usually on the first page for related key words. The problem is I think, is that it is a niche market - medical companies don't want to advertise because its travel related, travel companies don't want to advertise because its medical related. I've thought about turning ti into a charity - so far its all been my own personal investment - because there are patient education grants available - but I would still want to recoup the investment I have made. Any ideas would be very much appreciateed. Cheers.