I get emails from
funships@carnivalfunmail.com saying they're having a one day sale. I've not checked it out so it might be the same as the cruise lines. One thing I do know that can save you money on a cruise is if after you book it and know your destinations and times etc., you go online yourself and check out the area attractions which will usually be the same ones they will offer you through the cruise line. However, if you book the excursions yourself rather than go through the cruise line, you can save quite a bit of money (you cut out the middleman). You can grab a taxi at the dock and be at your destination before the people who booked through the cruise line and are therefore using the bus provided for them arrive there.
You can start checking with these places early on and that way you'll know how fast they are selling out for the time slot that you need. Make sure you keep an eye on weather reports before you book if the tickets are non-refundable.