For us it varies a lot. Sometimes we are just looking to sail a particular ship and little else matters (though cheaper is of course better). Other times it's about the itinerary. When possible, we try and work in holidays - specifically holidays that we have off from work, but that don't greatly increase the cost of sailings.
There are exceptions of course, like last month on Brilliance, when enjoying my drink at the R Bar I said, "oh, let's go to next cruise" and booked a sailing leaving in a couple weeks. I don't have impulse control problems.