Back in Seattle, SaturdayHouse originally had a contribution-based model. It seemed that people who paid also felt that they should participate because they had paid for the month.
Were this effect super-powerful, everyone would be fitness freaks, exercising every day because they had paid for their membership to the gym. Is this effect illusory or does it actually have some weight?
With planhack, people login, use it for a bit, then disappear for a month or two, then start using it again, and then fall off the wagon. So we apparently need a nudge to keep planning. I could send nag messages, however I know how I react to being nagged. ;)
Would asking people to donate some monthly amount help?