I travel a fair amount for work. Last year our whole family went on a trpk in an airplane. As we were going through security my wife was shocked as I grabbed a handful of the complimentary ziplock bags. I told her I always do this and you never know when you are going to need it. She laughed at me.
30 mintues into it 3 Hour fight, our youngest child (who at the time was 3 months) needed her diaper changed. As my wife was finishing she looked at me like where am I going to put this. Anticipating that ths was about to happen, I simultaiously was handing her one of the bags. That was the moment she saw the value in my grabbing the bags. There was a many times through out the trip that they came in use.
Another tip is skip the plastic packing bags and get zipper bags. I can get a link for them if anyone is interested. It is amazing how much stuff can be packed into them. Plus when you unpack or need to get something out it makes things much easier. Also when you get to the hotel or ship you just hand everyone their packing cube and it makes unpacking a breeze.