Mac OS X: what causes the spinning beach ball?

The window server will show the spinning wait cursor when the frontmost application, or the application that has a window under the mouse pointer, has not responded to events from the window server within a certain window of time.

To avoid the spinning wait cursor, an application needs to service events in a timely fashion. There's no way around this window server behavior, and for good reason: Applications on Mac OS X aren't ever supposed to be unresponsive to the user.