I think it's effective to warm the customer to the idea of paying for a full membership and the benefits of it. If you are strictly selling the option of a free sign-up then that's probably what they are going to do.
Comparing Free vrs. Paid member benefits, making it clear what you get by being a paid member is effective.
Giving some insight to what being a paid member is all about, some text...why it is worth it....ect.
People can get Free Water from the tap....but they buy it in a bottle all the time, that kind of thought process will open some more doors to the kind of marketing that might be effective as well.
Hope this helps, there is more...but that is what comes to mind after a long day
