Have you heard the story about the 300 million dollar button. Just by adding a button in the checkout process wher you did not havet o register to buy gave an online stor a boost of 300 million usd. Simply amazing how a small change can affect your online salse by so much.
When beginning your optimisation work always start backwards, meaning start with your checkout process first. This video from Google is a fantastic example on how many factors can kill your online sales. It is very funny!