Skimmed some of the others, however, please forgive any redundance.
Based upon experience.
1. Ditto on the guy who said to travel the day before if you need to fly to your departure location. Avoids stress and helps you be fully rested when the ship departs.
2. Arrive EARLY. I like to be on the first ones ON THE SHIP. You can get your bearings more easily with an emptier ship, and GET ANOTHER FREE MEAL!
3. Immediately after boarding, stop by the dining room; try to get friendly with someone, and find out your table assignment. If you are one of the first to put in for a change, you'll probably get it.
4. Remember that the staff can make your life merely pleasant, or a true JOY while on board. BE NICE TO EVERYONE, especially those who clean and perform basic labor functions. Tell them how much you appreciate them, and how much you hate passengers who complain about the service. Then you'll see what service really is! (This never fails).
5. Give out money at the beginning of the cruise. Let's face it, people work because they need $$$, have bills, family, etc....not because they are bored, and thought it might be fun to clean your dirty toilets while you're on vacation. I have never been told I couldn't do it, so usually start slipping out bills as soon as I spot who is capable of making my trip a good one. I often find that those who complain about service on ships are nasty, stingy passengers.
6. If you have a problem, DON'T GET NASTY with the staff...they are used to hearing everything, and having others be rude when things go wrong. The opposite behavior will get you everywhere. Then read No.s 4 & 5 above again, twice.
7. Take your own liquor on board. After you have done nos. 4 & 5, train your cabin steward to get you the same glasses used in the public bars. Make your own drinks in your cabin and use the savings to tip everyone else who will make your trip enjoyable.
8. Don't try to impress strangers at dinner. They might not have your jewelry or clothese, but might be able to buy the ship if they wanted.
9. If you think you will board late, and you have early seating, DON'T PACK YOUR DINNER CLOTHESE IN THE SUITCASE. This happened to me once, and, even though we were allowed to eat "come as we were, it was an awful experience".
10. Use premium dining. My mother says, "I already paid to get on here for the food in the dining room, why would I want to pay even more to eat". I say, "you are paying $30 for a meal you might have to spend $150 for at home".
11. If you have kids, remember that they are still YOUR CHILDREN when you are on vacation. A ship is neither a baby sitting service (unless you check in to that program) nor an excuse for your kids to act out of control and inconsiderate towards others.
12. Be considerate of others. You bought a ticket, not the entire ship. Everyone else paid to be there, too.
13. If you travel to a location where you don't speak the language, ONLY take cruiseline sponsored shore excursions. Otherwise, you are taking a big risk that isn't worth the money you might think you are saving.
14. If you tend to suffer from motion sickness, be prepared. If you use the patch, make sure to get more than what the doctor says you need. They come off in the shower and pool, and cannot be reapplied. If you are a serious sufferer (like me) then take dramamine the day before the trip leaves. Then, use only half a tablet at a time. Ask purser's desk at what point the water is likely to get rough. They know. Remember, much of the motion is controlled by underwater currents that are the same on every trip. You can use the patch and dramamine together, regardless of what you are told or read. Don't play "hero" with motion sickness...use the meds and you'll have a great time. It is a terrible thing to have your trip ruined because you have to spend 24 hours in bed!
15. Check if your personal med insurance covers you while traveling. If not, BUY SUPPLEMENTARY INSURANCE. EVERY CRUISE I hear others grumbling about their high infirmary bills. Things do happen!
16. Book as early as you possibly can. Book the best cabin within the cabin category you want (for example, most expensive outside, just below cheapest balcony, etc.). Upgrades do happen!
17. NEVER book a cabin adjoining an elevator lobby, NEVER!
18. Read reviews before you book; not all ships are created equal, and for good reason.
19. Book the first voyage of a new ship, if you can and don't care if the trip gets cancelled. You will probably get a free trip and reschedule.
20. Go with an open mind, and ready for exciting, new experiences. People are not all the same. No trip is perfect. If you're prepared, the best can be made of almost any situation.