We really liked this blog post from Brainsins and are reposting it to our blog with their approval: http://www.brainsins.com/en/blog/psychology-ecommerce-checkout/3751

Understanding human psychology is one of the best things you can do to boost your online sales, as we can use everything we know about psychology to engage customers, to make them buy faster or even to buy when they are not so sure to buy anything.