I always tell clients to pack half as much as they want to and it'll usually be twice as much as they need.
First, check with your airline because while some have a 50 pound limit for domestic flights, they'll often have a higher limit for internationally flights.
But with that said, depending on the cruise line we're using, I may not take a jacket opting instead for just a shirt and tie for dinner. That also means I don't have to take a pair of dress shoes.
We once did a trip where we toured around Europe for 2-1/2 weeks by train and then done a 12-night Baltic cruise. We actually managed to pack just one bag each. We also had a backpack that contained our cameras, netbook, and prescription drugs.
Some people insist on different outfits for every night and just take too much. We learned to pack smart, plan on double duty for some items, and use laundry services when available.
One thing we love are our convertible pants - they have a zippered leg on them so they can be worn as shorts or long pants. One pair, two purposes - works great. Also, we take layers - rain jacket that's very lightweight, along with a sweater that can be worn as a jacket for dinner if necessary.
I'm sure others will also have some great tips.