Totally depends on your service. Some services easily lend themselves to some sort of 'unlimited free/paid improved' model - e.g. GitHub's number of repos. Othe

Totally depends on your service. Some services easily lend themselves to some sort of 'unlimited free/paid improved' model - e.g. GitHub's number of repos. Others, however, are really difficult to shoehorn into that model, and a time-based trial is the best option - a 'content centric' service, for example.