I think part of it was just luck, and timing.
We started toying with the idea of a South America cruise after receiving a Cunard brochure. The cruise and ports of call sounded very exotic and interesting. The prices were prohibitive, for us-- over $7,000 per person. We started searching some of the discount cruise websites (esp. the travel section on AOL) in March of this year and studied and compared the fares, along with the cruise lines' sites. I think the economic slowdown at the beginning of the year had something to do with it too. We booked the cruise in April, a full 9 months prior to the cruise. It pays to book early. We got a category 4 stateroom (one level below veranda) for what is now being offered for an inside cabin.
We were going to purchase the cruise line's airfare package. Our early online investigations showed only a modest ($100-$200) savings by booking on our own. I did keep checking the online travel sites, and eventually I got the $1,200 fare. I guess you just have to keep checking those sites. The fares can, and do change constantly. We saved enough on the airfare to pay for all of the shore excursions we want to take.
The other thing we have going for us is the ability to be flexible in our schedule. The holiday cruise to the same ports is much more expensive than the one we booked, which begins on January 6, after the holiday crunch. The premium would also be applied to Carnaval time in Rio, when EVERYONE wants to be there. I guess I can sacrifice the crowds for a better value. I never was one to go with the crowds.
Call it savy, luck, or just good timing. I'm not sure which. Just be willing to spend some time investigating, and don't assume it's the final word. It doesn't cost anything to keep looking-- and it can save you a bundle!