Be sure you know how many formal, semi-formal and casual dinners you will have. Check with your cruise line if you are not sure. It's better to be over-dressed, than under-dressed. Some cruise lines do not allow jeans or shorts in the dining rooms even on casual nights (Holland America), so be sure you know the recommendation of your cruise line.
Pack clothes that you can mix and match - even with formal and casual dinners you can take four "dressy tops" that work well with two skirts and two dress pants. You will have 8-10 evenings covered. Pack just one formal dress (which you can wear two nights). Yes you can still take another fancy dress for two nights, and not over pack. By matching colors, you can reduce the number of shoes you need to take as well. Two pairs of dress shoes should be enough.
Remember, there are laundry facilities on board. Some ships have coin-op Laundromats, others you have to pay for them to do the laundry for you. Even though this can be pricey, you will only need to wash a few things along the way. Take eight or nine days of underclothes, and wash them at the half way point. Or less if you plan to wash clothes twice.
Layers, layers and more layers. By packing a sweatshirt or jean shirt, and a lightweight sweater, you can always cover up that short sleeve blouse while in colder temperatures. Leave the bulky jackets and heavy sweaters at home.
Three pairs of long pants (jeans or khakis for example) and three pairs of shorts should be enough. Four or five casual short sleeve shirts/blouses should do the trick. You wear them only during the day, and you can do a load of laundry during your journey. Besides, you are bound to find something to buy along the way!
No more than two swim suits please. And don't forget your cover-up, as you'll want it to walk through the ship or during lunch. No need to pack a beach towel, the ship will provide those by the pool. You may want a small tote bag to carry your suntan lotion, sun glasses, book, cards, and cabin key. Of course, buying one of these at your first port of call is always a nice (flat) souvenir to take home.
