one thing you can do is keep an eye on the various cruise travel agents - 1st4cruising, cruise.co.uk and all the others that advertise in the weekend papers. They very often have special offers that include free upgrades. The days cruise lines are pretty desperate to fill their ships. (Too late if you've already booked, though.)
Cruise lines usually allocate their upgrades on their websites and other travel related platforms as well on travel agencies database. Usually at the time of booking cruises people want to know about cabin upgrades that’s because finding your choice of cabin can be perk and can turn your good vacation into a great one. You can get such updates for longest coastline all over the world on travel agencies like http://www.koshertravelers.com/cruises/koshercruises.asp?expandable=0 and others that help you to plan your itineraries as per your need. Tell the agent to offer free upgrades for cruise journey throughout the year.