Some of the best deals are on the so-called repositioning cruises where cruise lines move ships from one continent to another with the change of seasons. Examples are some trans-Atlantic, trans-Pacific, and Panama Canal cruises. (Use your frequent-flyer miles for one-way flights to or from the other continent.)

I typically have purchased my cruises through vacationstogo.com, which often shares part of their commission with the passenger in the form of shipboard credits. I find their website very easy to use, but you do have to "register" on their website with an email address in order to see their offerings.

If you are flexible about your travel dates and are willing to play a game of chicken, some of the best deals appear anywhere from a couple of weeks to a couple of months before sailing date.