What worked for us was to do two versions of the program at the same time.

Version one had simple functionality. Version two had all the bells and whistles dreamt up by the programmers and the customer while we were working on version one. Those were added to a todo list for version two. They were not implemented in version one.

That kept up the flow of ideas but didn't disrupt the project. It made the programmers feel appreciated while giving us an outline for the next version so our competitors couldn't keep up.

What surprised me was that the whole team got into the spirit of the thing. When someone came to me with an idea, cries of "Version two! Version two!" echoed around the programming room.