First things first
Have a think about the things that annoy you when you're going through a checkout process. Next time you're ordering a book or a pair of shoes, focus your mind on how you would improve on someone else's procedure. Is it too confusing, too time-consuming and frustrating? If so, why? Take notes. You'll probably have seen some of the problems in our list.30 Checkout Optimization problems and how to solve them
1) There are too many hoops to jump throughYour aim should be to make the shopping experience as quick and simple as possible. Removing unnecessary steps can help improve conversions.
This case study from AB Tests shows how successful that can be.
2) You're forcing the customer to do something they really don't want to
Don't make it necessary to sign up for an account before your visitor can buy from you: you can ask them to do that afterwards. Provide a 'checkout as guest' option, like Walmart does:
Amazon make you sign in, but that's because they're enormous and already have a very good reputation.
Continue reading 30 reasons your customers aren't buying: How to optimize your shopping cart here