The mistake many people make is that they gorge themselves, eating beyond necessity and making unhealthy/unwise choices. Their reasoning: they want to get their money's worth. Then they complain about indigestion and weight gain. Go figure.
1. Try to eat only at mealtimes, very easily done because there's so much to be doing in between and 2. choose your vices wisely. I'm not a big dessert eater but I love bacon and bagels with cream cheese and lox, two things I only rarely eat at home. Also, I love a bottle of wine with dinner, particularly on vacation. So, in order to have my wine at dinner and get a daily fix of bacon or bagels with lox, I had to be a good girl and order fish at dinner, rather than steak, as often as possible, avoid the bread basket, particularly at dinner, and only order dessert when it really sounded/looked good.
The desserts aren't great on
DCL, so I wasn't missing much. The two desserts that I do recommend though are the crème brûlée at Lumières and the deep dish apple and cranberry pie served warm with ice cream at Animator's Palate (I hope I have that dining room right). And keep in mind that they also have three lighter fare choices on each evening menu, which tend to avoid sauces and include a good portion of steamed vegetables, like broccoli and cauliflower. That's always an option if you think you might be overdoing it. I never felt I needed to order from the lighter fare because there really is something for everyone on the dinner menu, even for people who watch what they eat.
The good news is that the portion sizes are reasonable, so you won't get huge quantities of tempting food placed before you. I ate everything on my plate, but would leave some of the carbs (rice, potato) behind because it makes me sleepy/lazy. My husband and I enjoyed our bottle of wine at every single dinner.
When I go on vacation, the one souvenir I don't want to bring back is 5-6 pounds of fat, so I can really relate to your concerns. I just got back last week, did not work out while on the ship (my body needed a no-gym rest too) and even with the wine every evening, no gym and a few sinful but yummy meals, I didn't gain or lose an ounce. I broke even.
Good luck and just make a plan and stick to it. If you feel yourself slipping, go on deck 9 and watch some of the people gorging on food from Topsiders...seeing those plates of pasta, chicken, seafood, mashed potatoes all mixed up together into "slop" will get you back on track in a flash. That's one thing I am certain of.