Have you considered an API that would allow other sites to add credit to user accounts automatically? I'm thinking specifically of sites adding GLBSE credits to users for promotions and compensation, instead of the paltry Bitcoin amounts they do now. It would be a good promotional situation for GLBSE.
There's a site called BTCTip
that tips Bitcoin from your account with them via a Tweet. I hooked Bets of Bitcoin up with them so they could combine what they do, and their userbases as well. If you combined your service with BTCTip (to add credit to another user's GLBSE account via Tweets) the resulting free publicity on peoples' Timelines would increase your trading volume immensely.
Were it me, I would combine that strategy with a referral link program - users earn a percentage of GLBSE's points from other user's they've referred with their link. The result would be more promotion, more trading volume and commission, and unlike placing ads it would only cost you a portion of money it had brought you anyway.