I just wanted to add that if you are going to charge for an app, make sure it does something unique that other free apps don't offer, otherwise you are not going to get anybody downloading and installing when there are other free options out there. A free trial is usually the best approach. I also liked the point that the app should be easy to install by incorporating Facebook ( or Twitter) signup options, which is really a one-click sign-up option - This is big since most people ( including me) simply move on when I am required to fill out a long form.