External Actionscript and calling external functions

I have not been able to figure out or find a good tutorial on how to use external actionscript or how to call functions located externally. What I have found in flash help and livedocs has not proved to be helpful and I have not managed to make it work. Has anybody seen anything?

Even a simple thing, such as creating an external function that tells a button to go to the next frame and how to call that in my flash file would be greatly helpful.