Just about everyone knows that you can use blends in Illustrator in order to generate the necessary steps in an animation. However, most folks also look at each item individually and only blend between two objects. Well, Illustrator's blends are far more powerful than that -- and when you create a group and make a blend between two groups, you can get a lot more animation for your dollar (or yen, or ruble, wherever you hail from).
As an example, if we wanted to create an animation of a sun setting behind a mountain, and that as the sun sets, the sky gets darker and the mountains darken as well, you could do so simply by creating a single group that contains all three elements: the sky, the mountains, and the sun. You could then duplicate the group and change the elements (change the colors of the sky and mountains and move the position of the sun) and then simply blend between the two groups, as you see below.
To complete the Flash animation, you don't even have to release the blend to layers. The Flash export dialog in Illustrator CS2 has an option which will automatically animate blends without your having to expand them.