What's the best way to find the lowest-cost "placeholder" cruise if we want to reserve one online (then change later to another itinerary)? I assume the 3-night cruises are the cheapest, but is there a time of year or specific itinerary that's typically cheaper, or a way to look them up by price? Also, does the timing of the "placeholder" cruise affect the timeframe of the eventual cruise we actually take? In other words, say the cheapest available cruise to book is for January 2014, but I don't intend to cruise until sometime in 2015 or 2016. Is it better to book a later placeholder? Are there restrictions on how far out one can actually take the trip and have the shipboard discount still be valid?