If you want someone to do something, you need to build what amounts to a funnel, or pathway. Help the reader identify the one thing they want, and then simplify and “narrow” the design and the text in order to focus on that one thing, and build energy and enthusiasm within the reader. Take away any distractions, visually or with words. Focus on the one thing.
And when you come to designing and writing the form or shopping cart, reduce the amount of friction by as much as you can. In other words, ask for as little information as you can, and reduce the number of pages to a minimum.
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And while we sold a little harder—and offered an incentive at the back end—the primary cause of the increase was almost certainly that we reduced the friction during the sign-up process. We offered more and asked for less.