I'm off in just under a month to an All Inclusive resort in Dom Rep. Its the 2nd holiday I've been on whilst on SW. I gained 2Lbs when I went just over a year ago to Mexico, which I think it pretty damn good for 2 weeks away!
Things to avoid
Fruit based drinks - they tend to be made with concentrates, are not real fruit and are horribly high in sugar.
Bread (obviously)
In a lot of Caribbean countries they have alot of American brands, so often the butter is sweetened (yuk!) as are all the sauces
There will be a whole island at breakfast full of pastries - AVOID THEY ARE THE WORK OF THE DEVIL. In fact, if you see any thin women piling their plates high from this station - chuck them in the pool cos they are obviously witches
Things to eat
Any grilled meats - there will always be some
Salads - they are always washed in filtered water now so its all good
Fruit - as above
If there are A La Carte Restaurants then my experience is that the portions are big enough to fill you and not so much you feel stupid full - I never worried when I was eating in one of these (most nights)
You will also find that the hotels are pretty big and you can walk off a lot of your food intake if you can be bothered in the heat!
Drinks wise- I just stick to spirit and diet coke with the occasional brightly coloured cocktail in there for good measure
Above all - ENJOY YOURSELF. Hope you have a fab time.