I have booked my last 3 cruises through Best Price Cruises online. They used to be cheaper than booking directly through the cruise line but now they are usually the same price but offer incentives.
This time, I got free travel insurance and coupons (for use in the gift shops, etc onboard). A few days after booking, Carnival sent an email about a one day sale they were having. I called Best Price Cruises back and asked if they could match the deal. They did... $100 off per person and the best available upgrade.... we moved up two decks and saved $200. Not bad!