A great deal of research has been done on shopping cart abandonment. Typically, when a hundred people start buying something online, of those who do not complete the purchase, seventy gave up somewhere while on the shopping cart pages.
Why? Too little energy. Too much friction.
As a formula, it is easy to visualize. In order to maximize the success of your site you need to increase the energy you transfer to your readers, and reduce the friction within the page or pages on which the reader has to do something.