Determining Interaction Sequences
Some web sites are so difficult to use that they have to offer something valuable for users to be prepared to tolerate the pain of driving their interfaces. If you want a web site to be used, it is worth considerable effort to ensure that the users can quickly navigate to the areas they are interested in and that they find it easy to interact with the site. This is especially important if the user has to fill out forms because forms in web-based interfaces are much less flexible in their interactions than their windowed counterparts, unless the web system also uses extensive client-side programming to manage interactions.
Try to arrange your logic so that when you validate a form's input you can present the form again, with messages about fields in error and the original form content still available. If you present an error message and ask the user to use her browser's "Back" button to correct the input, you are making it unnecessarily difficult to interact with your system. The only mandatory fields should be those that are truly indispensable to the operation of the system, and it is often a good idea to include a catch-all "other comments" text area to allow free-form user input.