If it is a paid membership. Have you considered a sign-up bonus! Maybe offering a small dollar amount for every sign-up which can be used in the site. Like if the membership fee is $25 then the sign-up bonus is $5 which can be used within the site. Or if it is a monthly fee then making the first month free. Or offer incentives with the site. Like for signing up they get such and such for free. If the site is about online advertising then they get their site submitted to 25 top search engines for free or a free e-book about online advertising. Or even teaming up with some of the bigger names in your targeted industry and getting great discounts for your users.
Just my thoughts! I never said it would work :) Hope this helps!