If you want to save money on hotel rooms, find those that are empty. The same is true of airline seats. Travel businesses cannot afford empties. They'll do whatever they can to fill them, even if it means deep discounts.
Cruises are no different. Each cruise line has its own Web site, and many of those actually allow you to view a floor plan of the ship and its cabins for a given itinerary.
Look at the cabins that remain unsold. Are there a lot of them? How soon does the trip begin? Chances are good that if the ship departs within a few weeks and there are a lot of available cabins, discounts are coming soon or may already be in place.
This happens more often than the cruise lines would prefer. A few years ago, these companies began trying to best one another by building bigger and bigger ships. Unfortunately, the timing for many of these projects did not match up well with a dropoff in travel following the September 11th attacks.
Don't expect the cruise lines to continue building giant ships for a while. You can expect to find some with empty cabins and deals in the making.