As crazy as it might sound I have my own Disney dining "review" website in which we try to review all the food we eat at WDW HOWEVER that being said overall, I try to eat low-fat and healthy as much as I can even in WDW. Makes it a little challenging to write about Disney food however it can be done.
SO here are some of my suggestions
First off, before you make ADRs (and you should make them), take a look at all of the menus and items on the menus of the restaurants you might be interested in dining at. allears.net has as well as here on the dis has a listing of menus for all meals that are updated on a regular basis. (this even goes for quick service so you know what to expect)
Items with sauces, dressings, etc should be ordered with those items on the side. Anything topped with cheese, try to omit the cheese. Most places will accommodate your requests.
I also tend to order appetizers or even 2 appetizers for dinner instead of an entree or instead of an app & entree. I try to order a salad and an app.
Try to have a least 1/2 of your meal include veggies preferably not cooked in butter or oil if it's not a salad.
If you must visit a buffet, stick with the fresh salad items, grilled items and items that aren't drenched in sauces or cheeses.
The Wave is a good example of a restaurant that uses a lot of fresh ingredients and even organic ingredients and most items are made to order but it doesn't mean they're always low-fat. You really need to ask what's in that specific item if you're not sure (ie. the grits at The Wave's breakfast are amazing however they are made with heavy cream, butter, cheese and bacon, a low-fat eater's nightmare LOL, so you really need to ask).
If you must eat something that is a little over the top in fat (like the grits), only eat 1/2 of that item or a very small portion and try to stay away from fried anything. (this is probably a common thing at the buffets-I take "tastes" as opposed to big portions).
One thing to keep in mind is when you're at a buffet or served a large portion of food, most people have the mentality of "getting your money's worth". This can be thought of in 2 ways...1-eating as much as you can until you feel like you're going to burst OR 2-eating enough to feel satisfied. Remember it's the same price whether you eat a little or a lot. I'd rather leave "satisfied" than feeling like I'm going to burst, kinda makes you feel like you wished you never ate any of it in the first place.

Hope it all makes sense.