Rather than doing this, I opted for a resource-based trial for historious. Free users get 300 bookmarks, if that's enough for them, that's great, if not, they can pay to get more. This way they don't put very much strain on the service but can still use it.

Another good idea, if your app can work with it, is to not let free users create any data. For example, for a website screenshot service, you could let free users access screenshots that paid users have created, but not create any of their own. This makes free users almost free to host but doesn't work for many apps.