If there are laundry rooms onboard you don't have to take as much clothing, just do laundry!
We cruise with 4 people, Dh, Self, Ds (8yrs old) and Dd (just turned 5).
We learned this after our first cruise. No one cares that you wear the same outfit once or twice. Makes life and luggage hauling MUCH easier!
We'll be taking this same cruise in July of "08 and only plan on packing 4 days worth of clothes for each of us. One big suitcase and we're done! Of course we'll still have a carryone for each person but just one large suitcase.
Have fun and can't wait to hear about the trip!