As well as your passport, always bring at least two copies of important documents like ticket, voucher and visa. Put one copy in checked luggage and the other in a wallet or purse for safekeeping. Leave a third copy at home with a trusted relative or friend. Check with the cruise agent for any other document requirements prior to departure.
The amount and type of clothing will vary depending on the cruise's destination, so do the research before departure. For example, Mediterranean cruises can be unexpectedly quite cool during the winter, while Bermuda cruises almost always feature an abrupt change in weather. Be prepared.
Take a change of underwear and socks for each day on the ship plus one. Find out if there are any formal events during the cruise and pack accordingly. Talk to the cruise agent about events on board the ship; being under- or overdressed for any situation is a mood-killer. Pack shoes for hiking (if applicable), walking on shore, rubber sandals for the pool area and any other footwear that may be needed on board or in port.
Think small when it comes to packing toiletries for a cruise. Buy the miniature size bottles (available virtually everywhere toiletries are sold) and leave the big ones at home. Do not take anything that is not absolutely necessary. If you need an unpacked toiletry item you can almost certainly buy it, or a comparable item, in the shops on board.
Avoid packing expensive electronics if necessary; leave mp3 players, personal DVD players, hand-held electronic games and other nonessential electronics at home. If traveling with a spouse, take only one cell phone and charger. If bringing a camera, pack it securely in carry-on luggage (never pack undeveloped film in checked bags).
Laptops, while sometimes needed for work, can add unnecessary weight to luggage and become a hindrance once on board. Many ships are beginning to offer Internet on board. Check with the cruise agent.
The most important part of traveling, whether on a cruise or flight, is preparation. Take prepaid phone cards (international and domestic, if traveling to a non-U.S. port of call), a guidebook, phrasebook (if traveling to a non-English-speaking locale), a flashlight (with batteries), a couple of resealable plastic baggies, extra doses of any prescription medication, a basic First Aid kit and the numbers of credit card and traveler's checks companies (in case they are lost or stolen).