Before you start packing, find out from the cruise line's website or your travel agent if your ship has self-serve laundry facilities. Some ships do, so you can pack half as much and wash clothes mid-cruise. Other ships only offer laundry and dry-cleaning services, which can be expensive.
Cotton is comfortable, but it takes forever to dry, so shorts, slacks and shirts in wrinkle-free synthetics are better. If you won't have access to laundry facilities, pack a small bottle of liquid detergent so you can do some washing in your cabin sink. Most ship showers have retractable clothes lines where you can hang items to dry. Pack two to three of each garment, preferably in mix-and-match colors. The same goes for underwear.
For men and women, khaki slacks can go from day to evening with a nice top or navy blazer. In daytime footwear, you need only comfortable walking shoes and a pair of sandals or flip-flops around the ship.
Bring two bathing suits so one can dry while you wear the other one, and a modest cover-up to wear while dining at the buffet. Bathing suits are usually not allowed in the dining rooms.
A nine-day cruise typically has two formal evenings unless the cruise line has done away with formal nights and gone completely casual. For women, a black skirt or slacks works every night with black shoes. Add two dressy tops, and you've covered formal nights. Or pack a simple black dress you can vary with jewelry, wraps or other accessories. For more casual nights, a few silky sundresses or mix-and-match tops (to wear over your black slacks) fill the bill. Men just need a dark suit, a dress shirt and two ties for formal nights. One or two pairs of casual slacks and a few collared shirts are fine for other nights. The rule is usually no shorts or T-shirts after 6 p.m.
If you have room in your suitcase and want to go all-out on formal nights, pack two gowns and a tuxedo with two shirts.
Other items you pack depend on where you're going. Alaskan cruises require warm layers, hats and gloves. Some European voyages may require dressing modestly, so pack a long-sleeved top to wear with slacks or a long skirt, a head scarf and closed shoes.
