Kane's best practice concerning Drupal Form's default Values.

exemple.php

<?php
function my_drupal_module_form_values($key, $form_state = array(), $default = NULL) {
  $settings = array(
    // Define here your form hierachies and default values
  );
  if (isset($form_state['values'])) {
    $settings = array_merge($settings, $form_state['values']);
  }
  if (isset($form_state['storage']['my_form_root_element'])) {
    $settings = array_merge($settings, $form_state['storage']);
  }
  $var = variable_get('my_form_root_element', FALSE);
  if (is_array($var)) {
    $settings = array_merge($settings, array('my_form_root_element' => $var));
  }

  $val = _extract_val($key, $settings);
  $val = $val ? $val : $default;

  return $val;
}

/**
 * Recursively walk through the data array to search elements based on the string|array $search.
 *
 * @param string|array $search
 *   The sub-element of $data you search
 * @param array $data
 *   The data to search in.
 *
 * @return mixed|NULL
 *   return element found or NULL. Don't forget to === for checking the result !
 */
function _extract_val($search, $data) {
  if (is_array($search)) {
    $key = key($search);
    $path = array_shift($search);
    if (isset($data[$key])) {
      if (isset($path[0])) {
        return _extract_val($path[0], $data[$key]);
      }
      else {
        $subkey = key($path);
        return _extract_val($path, $data[$key]);
      }
    }
    else {
      if (isset($data[$path[0]])) {
        return $data[$path[0]];
      }
    }
  }
  else {
    if (isset($data[$search])) {
      return $data[$search];
    }
  }
  return NULL;
}

function my_drupal_module_form($form, &$form_state) {

  // REALLY IMPORTANT: your form HAVE to be "tree" !
  $form = array(
    '#tree' => TRUE,
    'my_form_root_element'
  );

  $form['my_form_root_element']['my_fieldset'] = array(
    '#type' => 'fieldset',
    '#title' => t('My Fieldset Title'),
  );

  $form['my_form_root_element']['my_fieldset']['my_textfield'] = array(
    '#type' => 'textfield',
    '#title' => t('Textfield'),
    '#default_value' => my_drupal_module_form_values(
      array(
        'my_form_root_element' => array(
          'my_fieldset' => array(
            'my_textfield'
          )
        )
      ), $form_state, t('My Default Value')),
  );

  return $form;

}

gistfile1.aw

<?php
/**
 * Used to return a field's value, the default one if noone has been set in the $form_state.
 * Incidentally, this function embeds the default settings for all this module's forms.
 *
 * @param string|array $key
 *   The key of the searched value.
 * @param NULL|array $form_state
 *   The form state, if available, to extract submitted values.
 * @param NULL|mixed $default
 *   The default value. Caution ! MUSN'T be NULL if the form element 
 *   expect an array (checkboxes, select, etc.).
 *
 * @return mixed|NULL
 *   return element found or NULL. Don't forget to === for checking the result !
 */
function HOOK_form_values($key, $form_state = array(), $default = NULL) {
  $settings = array(
    // Define here your form hierachies and default values
  );
  if (isset($form_state['values'])) {
    $settings = array_merge($settings, $form_state['values']);
  }
  if (isset($form_state['storage']['my_form_root_element'])) {
    $settings = array_merge($settings, $form_state['storage']);
  }
  $var = variable_get('my_form_root_element', FALSE);
  if (is_array($var)) {
    $settings = array_merge($settings, array('my_form_root_element' => $var));
  }

  return _extract_val($key, $settings);
}

/**
 * Recursively walk through the data array to search elements based on the string|array $search.
 *
 * @param string|array $search
 *   The sub-element of $data you search
 * @param array $data
 *   The data to search in.
 *
 * @return mixed|NULL
 *   return element found or NULL. Don't forget to === for checking the result !
 */
function _extract_val($search, $data) {
  if (is_array($search)) {
    $key = key($search);
    $path = array_shift($search);
    if (isset($data[$key])) {
      if (isset($path[0])) {
        return _extract_val($path[0], $data[$key]);
      }
      else {
        $subkey = key($path);
        return _extract_val($path, $data[$key]);
      }
    }
    else {
      if (isset($data[$path[0]])) {
        return $data[$path[0]];
      }
    }
  }
  else {
    if (isset($data[$search])) {
      return $data[$search];
    }
  }
  return NULL;
}