Theming a form in Drupal 6
Form theming in drupal 6 is quite flexible. One of the nicest tricks is the ability to theme the form in stages. Calling drupal_render on a part of the form will not only return the rendered version of it, but it will also mark that part as rendered so that when you call the final drupal_render over the whole form, it will not re-render the parts already rendered.
Remember that in order for the form to function correctly there are some hidden fields in every form that must be added to the form. Also drupal needs to generate the form tag. Therefore it is my advise, after doing the custom rendering of the parts you want, add a general render of the form. this will reduce the chance of problems with form submission.
Special thanks to Rolf van de Krol, for this tip.