I am finding a lot of restaurants have a published "diet plate" upon request. Mimis Cafe is my most recent example. I am not sure why they don't offer this right from the beginning for everyone, but I have found several places that will do so if you ask. You can also ask for your order to be modified-- less cheese, sauces on the side, etc.
Most restaurants seem to serve family-sized portions. Macaroni Grill and Claim Jumper are two that way over-feed their guests. I usually share a meal with whomever I am with, or I order my "to go" box right at the beginning and put away the portion I shouldn't be eating. Somehow when you are sitting there talking you eat everything on your plate and with the portions that are served that is like eating two meals instead of one.