Follow up to my blog post: Drupal – adding debug messages to your custom module – Improved - http://pages.uoregon.edu/vid/?p=2854 This is the code snippets I use to add debugging functionality to my custom Drupal modules.

gistfile1.php

Main form fields amendment:   
<?
function mymodule_admin_settings($form, &$form_state) { //this is a funky comment
  #...
  $form = array_merge($form, _mymodule_debug_form()); //append settings form with debug fields
  return system_settings_form($form);
}//end function mymodule_admin_settings($form, &$form_state)
?>

<?
/**
 * Page callback for admin/settings/mymodule.
 *
 * @see mymodule_menu()
 */
function mymodule_admin_settings($form, &$form_state) {
  mymodule_debug_log('Running function mymodule_admin_settings');
  // Checkbox to enable or disable the tab.
  $form['mymodule_your_settings'] = array(
    '#type' => 'checkbox',
    '#default_value' => variable_get('mymodule_enabled', FALSE),
    '#title' => t('Enable tab'),
    '#description' => t('Activate the tab.'),
  );
  
  //include the debug form fields
  $form = array_merge($form, _mymodule_debug_form());
  
  return system_settings_form($form);
}

?>


debug form fields that get included above:
<?
/**
 * Helper to produce mymodule debug configuration.
 *
 */
function _mymodule_debug_form() {
  mymodule_debug_log('Running function _mymodule_debug_form');
  // Debugging - fieldset for debug related fields
  $form['mymodule_debug'] = array(
    '#type' => 'fieldset',
    '#title' => t('Debugging'),
    '#collapsible' => TRUE,
    '#collapsed' => FALSE,
  );

    $form['mymodule_debug']['mymodule_debug_message'] = array(
      '#type' => 'checkbox',
      '#title' => t('Enable debugging message feedback'),
      '#description' => t('Displays feedback in Drupal Messages'),
      '#default_value' => variable_get('mymodule_debug_message', FALSE),
      '#required' => FALSE
    );

    $form['mymodule_debug']['mymodule_debug_reporting'] = array(
      '#type' => 'checkbox',
      '#title' => t('Enable watchdog debugging feedback'),
      '#description' => t('Displays feedback messages in the Recent Log Entries (watchdog) Report'),
      '#default_value' => variable_get('mymodule_debug_reporting', FALSE),
      '#required' => FALSE
    );

  return $form;
} //function _mymodule_debug_form()


?>

The actual debug log function
<?
/**
 * Helper to output debug messages.
 * 
 * mymodule_debug_log([$errMsgArr(array), $errLabel(string)='mymodule', $errSeverity(string)='status|warning|error', $force(bool)=FALSE])
 *
 */
function mymodule_debug_log($errMsgArr='', $errLabel='mymodule', $errSeverity='status', $force=FALSE) {
  $backtrace = debug_backtrace();
  $watchdog_levels=watchdog_severity_levels();
  if ( ! is_array($errMsgArr)){
    $errMsgArr = array($errMsgArr);
  }// end if ( ! is_array($errMsgArr) )
  foreach ( $errMsgArr as $errMsg ) {
    $spacer = ($errMsg)?' :: ':'';
    $errMsg = $errMsg . $spacer .'Line: ' . $backtrace[0]["line"] . '. Function: ' . $backtrace[1]["function"] . '. File: ' . array_pop(explode("/",$backtrace[0]["file"]));
    if ( variable_get('uoasymptote_debug_message', FALSE) || $force ){
      drupal_set_message(t($errMsg), $errSeverity);
    }// end if
    if ( variable_get('uoasymptote_debug_reporting', FALSE) || $force ) {
      #watchdog($errLabel,$errMsgArr);
      watchdog($errLabel,$errMsg,array(),array_search($errSeverity,$watchdog_levels));
    }// end if
  }// end loop
  unset($backtrace);
}// function _uoasymptote_debug_form()
?>

Example usage in the form:
<?
function mymodule_form_validate($form, &$form_state) {
  mymodule_debug_log(); //outputs: Line: 109. Function: mymodule_form_validate. File: mymodule.module
	...
}

...
if ( ! $path ) {
  //force this debug message to the screen
  mymodule_debug_log('Notice: '. mymodule_library_location_stmt(), 'mymodule', 'error', TRUE);
} else {
  ...
}
?>