My "Forms that work" web site has several articles on this topic.
The key concept is to think about the three layers of the form: relationship, converation and appearance.
"Relationship" is my term for all the reasons why the users is filling in the form and why the organisation is publishing it. Are these in alignment? Are you asking for the minimum of information, and do the users expect to be asked those questions?
"Conversation" is my term for the question-and-answer process that is an inevitable part of a form. Do the questions make sense to the users? Can they find the answers? Have you designed validations that respect their efforts and the real-world range of the answers?
"Appearance" is about making the form look good - often the part of forms design that attracts the most time and attention, but in fact users rarely bail out of forms because they are worried about fine details like whether the labels are above or to the left of the boxes. If the form looks neat and tidy, it will probably be OK.