You can also use the 'DOCUMENT_ROOT' server variable:
include $_SERVER['DOCUMENT_ROOT'] . '/includes/myfile.php';
If not, you can always do as HalfaBee suggests and modify the include_path PHP directive to include your 'includes' directory in the default path. If you don't have access to the php.ini file (or don't want to make this change global), you can use the 'php_value' command in a .htaccess file to achieve the same result.