I'm back
Well I run a lot of large forums. If you want members to come back then remind them to come back with a monthly email newsletter or bi-monthly emails to inactive members saying HEY WE MISS YOU.... this will get the active number of users back up then you can always end the newsletter with benifits of being a charter member to help boost subscriptions. It is also very easy to setup a paypal link to make becoming a charter member very easy. Here is my last idea make a paypal subscription link that auto renews every 12 months. If mine auto renewed every 12 months I wouldn't of cared.
just my 2 or 3 cents.... worth