Colesworthy - if you're sitting on a fully engaged wealthy demographic that considers your offering sufficiently valuable to actually pay for it, then your membership is every luxury and premium lifestyle brand's dream.
Without knowing any more than the minimal details in your question, I would recommend that you target luxury goods/premium lifestyle brand advertisers with the aim of developing creative commercial partnerships for your site, ie derive a revenue stream, not necessarily simply from pure advertising, but from ways of jointly delivering mutual value to your membership, by integrating those brands with your site offering to provide a seamlessly engaging online experience, via customized promotions or interactive entertainment.
You cite frequency of usage, which is also a great selling point, and provides the opportunity to develop loyalty programs with commercial partners.
I'm not sure I can add much more without knowing more about the nature of your site, but hope that helps.