organizing private functions

[eluser]Insteps[/eluser]
I have books, authors, sellers controllers. In controllers I have some private functions like

Code:

_setBookByAuthor($a) {
...
}

The function calls a common model function, and some helper functions for text manipulations and sets appropriate output in an array.

Incidentally this function became a bit generic and could be used from all controllers.

Is there a way of defining these kind of functions once so maintenance becomes easier?

Thanks.