do your homework, go to the website check out the ship. Go to the library and get either Frommer's or Fordor's travel guides to your ports. Ask questions on CruiseMates.
The most important thing right now for them is to file for their passports, do it now before the end of the year rush to get them when the new regulations go into effect for 2006.
Check out the Practical Advice section on this site.
Read all the ship reviews you can find, both professional and reader reviews.
Go a day early, buy insurance.
Pack. Then take half the stuff you packed out of the suitcase!
In your carry on put meds, toiletries, swim suit or whatever you tend to wear after boarding, and your dinner clothes for the first evening in case luggage delivery is delayed.
Wear something cute for the welcome aboard photo, get an umbrella drink and enjoy!
Ditto to those who say "get down a day or 2 early". It is worth the $$$ in a moderately priced hotel. Also, I have only let the dock porters handle my luggage 1 time....that was the last time too. Didn't see it at my room until about 10pm that night. Since then I carry my own. I am unpacked and on deck with 3 drinks down before most are coming up the gang plank. However if this is an "older" couple, this might not work for them. To each their own. Hope the trip is a memorable one as was our first !!!!