http://blog.ometria.com/ecommerce-webform-best-practice?utm_content=buffere2d66&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer

1) Mark optional fields rather than required ones
2) Use in-line form validation
3) Use smart defaults
4) Top- or right-align your labels
5) De-clutter your forms with information popups
6) Be clear about what you’re going to do with the information you ask for
7) Don’t make users remember yet another username
8) Let people know what stage of the form-filling process they’re at
9) Test your CAPTCHAS
10) Make your CTAs clear
11) Format your payment details fields so they're the same as they appear on a customer’s card
12) Set the delivery address as default billing address