[FrankHudson]

As has already been mentioned, there's no international treaty that says you have to have a C section.

But if you want one, and are stumped, there are little rule/idea things you can try just to see if something works for a piece: play another section again, but reverse the cadence. Modulate to a new key, including the old favorite switching major to minor or vice versa. The same chord progression, but change one chord. Change the rhythm, not the harmony. Drop the top line melody and just play the bass/lower end notes for a section....There are more of course. There's even the Brian Eno Oblique Stategy cards which tend to be more philosophical/approach-ish.

And when you try one of these things it may sound absolutely wrong, but reacting to that can still break you out of your rut in a different direction.