Anywhere you are unconditionally including a class file, use require_once(). Anywhere you are conditionally including a class file (for example, factory methods), use include_once(). Either of these will ensure that class files are included only once. They share the same file list, so you don't need to worry about mixing them - a file included with require_once() will not be included again by include_once().

Note: include_once() and require_once() are statements, not functions. You don't need parentheses around the file name to be included.

When including code from the same directory or a sub-directory, start the file path with ".":

include_once ./includes/mymodule_formatting.inc

In Drupal 7.x and later versions, use DRUPAL_ROOT:

require_once DRUPAL_ROOT . '/' . variable_get('cache_inc', 'includes/cache.inc');