The key to booking the lowest priced Disney cruise is to book as early as possible, generally the first few hours when the itineraries are released. A Disney Cruise specialist travel agent will be able to help you with this. Also, while aboard the ship, make a future cruise booking on a "dummy sailing" which can be extended indefinitely. This guarantees a further discount.
The people here on Flyertalk and also Disboards have lots of good tips about DCL.