First, pack the basics for both men and women: one or two swimsuits (and a cover-up for women), two pairs of walking shorts, one pair of jeans, a few blouses/polo shirts, one or two lightweight pairs of slacks/skirts, undergarments, a good pair of walking shoes, a few pairs of socks/stockings (non-cotton), some water socks, a couple of white T-shirts (non-cotton), raincoat or plastic poncho, a hat, sunglasses and sunscreen. Check with your cruise director to see where the ship will dock for off-boat excursions. Some islands and countries prohibit racy outfits like short-shorts and halter tops. It is best to tilt in the conservative direction.
Weather conditions can change dramatically, especially if you are taking a winter cruise or visiting a location like Alaska. Pack a lightweight to heavy jacket, one sweater, one pair of knit or wool pants, heavy socks and one pair of boots or high-top shoes. You may also want to pack a scarf or winter hat. It is best to be prepared for cold conditions instead of suffering through through them.
Most cruise ships have at least one formal dinner and several casual dinners. For the casual nights, bring dress slacks/skirts, a couple of blouses/dress shirts and a sport jacket (ties are usually optional). For formal dinners, a cocktail dress or long gown and a dark suit are appropriate. A tuxedo is optional on most cruises, but check with the cruise director. If it is required, a tuxedo can be rented before you sail; check with the cruise line for more information.
