Split meals--even counter service. At some of the table service meals I don't even order an entree sometimes. If you can't split a meal, decide what is a portion, and get the rest off your plate either in a to go box or on another plate.
At buffets, put your fork down between bites. Decide how many plates you will have, ex: a plate of fruit, salad, a plate of main course, a plate of dessert.
Plan your treats. If we want ice cream, sometimes that would be our lunch. Would I do that home, no but it works better than eating a big lunch and having a treat. If you have a certain Disney treat you love, have a plan for it. Share treats. A little is great, a lot just makes you feel yucky.
Watch thinks like fries, chips, popcorn, etc. They are my personal weakness, but I have found that if I put how many I want in front of me, and move the rest I will stick to it.
Try to eat healthy at least 90 percent of the time. You will be walking a lot, so a little splurge is ok, but going overboard will just make you feel yucky.
If you can walk, do it. Skip the boat, bus, etc if you can walk instead. If you exercise regularly, try to get a few workouts in. We took two runs on our last trip.
Try to mostly drink water. All those specialty drinks and soda have tons of calories.
Our last trip we stayed concierge and I only came back with a 3 pound gain, and it was gone in less than a week. You can still enjoy yourself and eat lots of tasty food, moderation is just the key.
Have a great trip!