Make a last-minute reservation... Go to the Princess web site under "Specials". Right now they've got a 7-day Caribbean on March 28th for as low as $499. Those deals are almost always out there.
Early June in the Caribbean is pretty good too... right before all the kids get out of school. ...and the departure ports are much less crowded then too, since there are very few ships doing the route at that time.
The peak of the hurricane season tends to have lower prices too...