This article by Nick Usborne, Flywheels, Kinetic Energy, and Friction, beautifully describes the behaviour of customers in the lead up to buying and in the act of buying. It shows what you as the vendor can do to help, or get in the way of your customers.

Whilst Nick's article focuses on websites, I believe the concepts therein apply equally to all the customer touchpoints in any business; such as forms, shop/office layout, phone systems, manuals and catalogues.

Thanks Stil for pointing out this article.