[development] Extending Modules

I very much expected to find "best practices" for extending modules, but I haven't. Does anyone have any pointers to resources or advice? It's great that Drupal itself is modular, but if I want to add a little something to a module that already does 90% of what I want, it seems there ought to be a standard way to do this without forking it. Wrapping a module I can imagine how to do, and overriding or adding functions as needed, but it seems there could be a way to do this without having to define every function and pass it through. At least I hope to find recommendations on how to do this. Thanks for any help, Ben Melançon of Agaric Design http://agaricdesign.com/