Book airfare separate from the cruise.

Check out Cruise Critic for loads and loads of advice.

Once you settle upon which ship and which sailing you'd like to try, use Cruisecompete.com to find the best deal.

Consider, if possible, sailing at a slightly different time of year. As noted, Christmas/New Year's sailings are some of the most expensive ones out there. Even just moving your trip a week earlier in December or a week into January will save you a lot of money.