Implementing techniques like field highlighting (so the user can easily identify the problem field) and inline validation (so the user does not have to scroll to the top of the page to read the error message) can enhance the usability of a web form.
Avoid complexity
Nobody likes filling in forms at the best of times and completing an online form is no different. Consider implementing features that reduce the effort of filling in the form, such as a postcode lookup/address auto fill, drop-downs or even pre-populate fields with content stored in the database.