Like several others have mentioned, I think the Premium Member model is a winner. Folks who are willing to shell out a few bucks a year, don’t see the ads, while non-paying members (and lurkers) do.
This is the same model that the Delphi Forums network uses. On their network, not only do Premium Members not see the ads, they also have a bunch of cool posting functions that non-paying members don’t have.
Altogether, it works very well.
Working the FReepathon you hear stories. People out of work, one lost their house and was living with relatives. She sent $3. Dear one, she WAS a "premium member". And she's not the only one.