A technique that I’m finding quite useful during module development is to create a “stub” function for use as a generic interface callback for menu items that I want to define, but haven’t yet written callbacks for. For example, if I were building a Drupal 5 module called widgetmaster, I would define:

function _widgetmaster_stub($message = "This feature is still
  in development.") {
  return '<em>'. t($message) .'</em>;
}

So if I’m planning an admin section for my module but haven’t gotten around to implementing it yet, I can at least build out the menu so I (or a client) can click around and make sure the interaction and flow makes sense… for example:

/**
 * Implementation of hook_menu().
 */
function widgetmaster_menu($may_cache) {
  $items = array();
  $access = user_access('access administration pages');
  if ($may_cache) {
    $items[] = array(
      'title' => t('Widgetmaster Settings'),
      'path' => 'admin/settings/widgetmaster',
      'callback' => '_widgetmaster_stub',
      'type' => MENU_NORMAL_ITEM,
      'access' => $access
    );

    $items[] = array(
      'title' => t('Overview'),
      'path' => 'admin/settings/widgetmaster/overview',
      'callback' => '_widgetmaster_stub',
      'type' => MENU_DEFAULT_LOCAL_TASK,
      'access' => $access
    );

    $items[] = array(
      'title' => t('Advanced'),
      'path' => 'admin/settings/widgetmaster/requiredfields',
      'callback' => '_widgetmaster_stub',
      'type' => MENU_LOCAL_TASK,
      'access' => $access
    );
}

I can also leave myself specific notes as to what a particular menu callback is supposed to do:

...

$items[] = array(
  'title' => t('Advanced'),
  'path' => 'admin/settings/widgetmaster/requiredfields',
  'callback' => '_widgetmaster_stub',
  'callback arguments' => array('Select advanced options from
          the widgetmaster_frob table and display as a set of               
          checkboxes');
  'type' => MENU_LOCAL_TASK,
  'access' => $access
);

...

Usage needn’t be limited to menu callbacks, either; it’s handy wherever you know you’ll eventually be defining a function that returns a string.

I’m finding it very helpful to have these stub interfaces in place early on in development. It helps fine-tune interaction, and it also serves as a contextual TODO list.