DCL will offer Florida Resident rates and Military rates when it needs to fill rooms, but these are usually offered only a few months before.
They have been known to offer Kids Go Free offers, but only on itineraries that aren't selling well.
Your best bet is to check the MouseSavers discounts page for DCL. You will find the occasional Onboard Credit Offer.
Another tip - when rates are first released for sailings they are at their lowest; for example, rates for the first half of 2012 were released a few weeks ago. As the ship fills, and as the sail dates come closer, the prices will rise.
Also, just like WDW seasons, DCL will offer its rates based on seasons - for example, Spring Break/Easter will be the highest priced, late August into September usually are the lowest priced...
In general, you won't find deep discounts like you will with WDW room offers as DCL generally sails full, and they only have 2 ships, with the 3rd about to have its maiden voyages in January.
Supply and demand.