Usability and HTML Forms is one of the best articles I've seen on the subject. Lots of "aha" moments here.

These three examples demonstrate three different relationships between users and forms. The tax form is obligatory. It's a pain but has to be filled in. An e-commerce form is filled in by choice in a shopping environment where there are alternatives. In most cases the user can abandon the form and try a different company. A registration form at an early stage on a site creates the most fragile relationship. There are lots of alternatives and the reward is unclear - who knows what's inside?