What's the optimal flow for a check-out process?

I like a extension of Magento as a good example One Step Checkout. It check all field gives visual confirmation of correct and incorrect fields and update shopping card dynamically so you see that all is going well with your order.

I am all for One Page Checkout over Multiple pages.

Good examples and comments from everyone. One thing to add is to perhaps create it like a wizard (like someone else said), but not have a back on each page as that will allow your customer to mess up their order (obvisouly, it depends on how easy/complex your product is to order). But regardless, you can build it such that you drive the customer from 1 page to the next, all the way to the end, and then show them a review order page, which includes links to edit any of the sections. (Try to configure a laptop on HP or Dell for example). This allows for a cleaner workflow/design, and the customer is not lost in the multitude of pages. But again, number of pages should be kept to minimum and try to categorize the pages: Page 1: Select product/features, etc. Page 2: Registration, Shipping Address, etc., Page 3: Shipping, Payment, etc. Page 4: Review and Checkout

Check out process is an entire science in itself.

There are some general rules such as you loose 5% for every page and so on.

But they are really not interesting unless you are amazon and it become a numbers game.

What matters is how little friction you create in the process and how understandable it is for your user to buy your product.

Two links

A review of the new apple checkout process with some interesting flaws http://www.lukew.com/ff/entry.asp?968

The PC over them all. They have a pretty good checkout process despite it's complexity. http://alienware.com/

But remember for most companies it's about friction and not page numbers. That is unless you are Google or Amazon.

As a general rule of thumb, the amount of steps needed to complete a purchase is inversely proportional to the conversion rate.

Not sure about multiple pages of data entry unless:

• A) The user knows they are on page 1 of 2, 2 of 2, etc.
• B) Verify my entries before going to the next page.
• C) Organize the pages so there would be little need to go back and forth.
• D) Have a really good reason why I have to enter so much information to buy something off of your website (Excluding informaton about products as I enter them into shopping cart.).

Thank you for being cognizant about your users during the checkout process. You may want to track users who cancel the purchase process after the first page. That would be a strong indication that this is a bad idea.