I picked a particular cruise, ship, cabin and dates then went shopping. I checked almost every web site and a few TA's locally. If you have time, you can even watch that sailing go down in price. I waited and booked our cruise 2 months in advance. It's a gamble if your dates are set in stone. In my case, I could have waited to book until today (we leave Sunday)! There are still available cabins on my ship & they're really dropping in price.
Use the same techinque to search for the flights as well. They don't go down in price but every site seems to have a different price. TIP, don't book your return flight before noon on the day you return. It makes it really tough to catch your flight home.