I don't have a solution or a suggestion about making money on sites like Second Life, but I am aware of that community and know that others, like you, have been able to make some
money on it.
I haven't used Second Life - I have only watched over someone else's shoulder, but it seems to me that to truly make some money, you might have to create some kind of application or "product" that Second Life users will see as a "must have" for their Second Life presence. For example, some kind of super-duper clothing, or a tool or device their SL avatar can use to do something amazing.
It would have to be something truly innovative. I would have to go there to get some ideas (but that won't be any time soon, unfortunately).