Cabin-Make sure you request a cabin that is away from the elevators so you don't have to deal with the foot traffic if you are wanting to get some sleep.  Your travel advisor can help with your location.
Mediciations-Make sure you bring your meds if you need stuff for motion, allergies, colds or meds for the kids.  Options on the ship are limited and not cheap.
Shoes-Make sure you pack comfortable shoes.  Everyone likes to dress up for dinner and shows but climbing stairs or walking on some surfaces are not real comfortable with high heels. You don't want to lose your balance on a moving ship.
Suscreen-It is always better to have TOO much than not enough.  Consider some aloe vera as well in case you get sun burned.
Put phone in airplane mode-It is a MUST! If you don't put your phone on airplane mode once you are at sea you will have a nice big phone bill when you return.
Ship newsletters-These are in your cabin each evening. Please review them so you know what is going for the next day.  This way you can plan your day out easier and not miss some activities or shows you want to participate in.
Excursions-Most cruisers believe they will "Miss the boat" if they don't book excursions with the cruise line.  This is actually not true. Many times the cruise line use the same vendors. The advantage the tour operator has is that their groups are smaller, more personalized, better service and can be cheaper. Their business focuses on excursions so they comply with the cruise ships's schedule. I always offer other vendors as I use them myself.
Carry on-Make sure you have your important documents(passport and boarding pass), change of clothes, medications, sunscreen and anything else you ABSOLUTELY need once you are on the ship.

Luggage Survival Tips
 
Many travelers at one point or another may encounter dealing with lost luggage.  Your luggage didn't make it to your arrive airport with you, it got routed on a different plane, it got left behind at the hotel, etc. Here are some tips to help you with luggage that are MIA.

1. Take a picture of your luggage. This allows you to show a picture of your luggage instead of trying to describe it to an agent. 

2. Split clothes 50-50.  If you are traveling with someone else consider splitting your clothes amongst each others bags. This way if your luggage gets lost you have some clothes.  It is unlikely the airline will lose both but it is better to be prepared.

3. When reporting the MIA luggage to the service desk you want to describe items that stand out and that will be recognizeable. (ex: red hat, yellow tennis shoes, floral shorts)

4. Provide your phone number and address of your hotel if your luggage is lost while traveling. If it is on your way home then a phone number should be good.  Ask for a copy of the report and a number where you can follow-up.

5. Ask for a reimbursement if this occurs while traveling. Most airlines will cover the necessities like clothing and toiletries while you wait for your luggage. If you are arriving home after a trip it is not as likely. Consider purchasing travel insurance which will cover situations like this.

6. Inform your hotel about your situation if you are just arriving in your destination. Let the front desk know as they may be able to get your bag back faster if they contact the airline especially if you are not fluent in the language.