How can I prepare for feature creep during preproduction?

Feature creep is when you are in the production phase and decide "It would be cool if this were a feature!" That adds n hours to the development time which were not accounted for, and unless you are Blizzard, do not have the time for. You cannot anticipate it being a cool feature during preproduction, because the feature is only discovered during production.

What can I do during preproduction to account for or prevent or prepare for feature creep?

Should I allow feature creep and cut less desired features? Or will this create a problems since the architecture is not designed for the new features? Should I say no features unless they fit into the existing architecture?

I know this is a broad question, but I hope it doesn't get closed as such. This is a major issue in game development, and should be considered.