// Inside mymodule.module
module_load_include('module', 'mymodule', 'mymodule.menu.inc');


// Inside mymodule.menu.inc
function mymodule_menu() {
 $items['pages'] = array(
   'title' => 'Test',
   'description' => 'This is a test of having hook_menu() outside of the .module file',
   'page callback' => '_mymodule_callback_string',
   'file' => 'mymodule.callback.string.inc',
   'file path' => drupal_get_path('module', 'mymodule'),
   'access callback' => TRUE,
 );

 return $items;
}