I try to avoid self-referencing, but since it does answer your question, here's my idea creation/testing thought process[1] and a walkthrough of an example I'm currently working on[2].

Joel (of Buffer) also did an awesome write-up of how he validated that idea[3], which is probably the best practice for freemium or subscription ideas.

[1] http://thestartuptoolkit.com/blog/2011/12/how-i-come-up-with...

[2] http://thestartuptoolkit.com/blog/2012/02/an-example-of-how-...

[3] http://blog.bufferapp.com/idea-to-paying-customers-in-7-week...