First few cruises, I gained a pound every 2 or 3 days. Since then, I have managed to maintain or lose weight by following some guidelines:
1. Make some good choices at the MDRs. Note that I said "some," not "only." So one night I might choose a high calorie appetizer, but the next two nights I might go for a salad and minimal dressing. It helps a lot that I have tried most of the items, so I don't feel the need to try everything now.
2. Decrease the snacking / trips to the quick service dining spots. Also, when I go there, I ask them to only give me a half order of fries. At first, I thought they gave me a full order. Later, I ordered a full order and discovered they had been giving me the half order I requested! That's a lot of fries. Of course, cutting the fries by half only works if you don't double the number of times you go there.
3. Must do an hour of cardio at the fitness center on any sea day.
4. Only allowed to use the stairs on any sea day.
5. Be active when in port: walk places, discover things.