Everyone always asks me what to pack for a cruise. I did a blog recently on this as well http://travelonadream.wordpress.com/2010/10/29/cruise-packing-101/ so I won't go into the details I included there. What is important to know is what type of dress code your cruise requires. Norwegian has mostly eliminated formal nights while Cunard has a number of formal nights on their sailings. In fact, most nights Cunard even has a ball to attend with a variety of themes!
Before you start packing, you will want to find out if there are formal nights, semi-formal nights, pirate nights (like on Disney), other themed nights so that you can either make alternate plans for those nights or pack accordingly. On Cunard, the dress code for formal night is formal dress. If you do not wear formalwear, you won't be allowed to eat in the dining room.
Another thing to keep in mind is that many of the cruise lines will have photographers available most nights around dinner and the shows to take photos of you with various backgrounds. There are no sitting fees and you only pay if you purchase any photos. This would be an inexpensive way to get a family photo or picture for your holiday cards (of course it's not that inexpensive when you add in the cost of the cruise along with the cost of the photos!).