It depends on your choice of language and some specific implementation details....and whether you are more interested in the user interface side, the data model

It depends on your choice of language and some specific implementation details....and whether you are more interested in the user interface side, the data modeling, or both. There's a range of approaches to motivational systems, and I think in 2010 we'll see some real innovation in this area. For instance, I think Gowalla is doing a particularly great job.