Smashing Magazine has a great two part article on common patterns in form design. The author evaluates 100 forms from popular sites and gives a break down on all the similarities they find. Its essential for any website to have forms that are concise and easy to understand. After working in the online advertising industry for a while I can attest that a great form will drastically improve completion rates. Something as simple as changing the alignment of a few fields can effect the users behavior. Head over to Smashing and check it out!