Before you go out, look at the nutritional info on their website(s). Make a short list of items that meet your nutritional goals and only look at your list, instead of the whole menu. If you have a very favorite dish that's very indulgent, you can also look up the nutritional info and figure out how much of it you want to eat (say 1/2 or 1/3 of the portion) and then box the rest up right away.

Whenever I go out to eat. I look at their menu and nutrition online if possible and find something that will fit and I actually want to eat. If they are a small mom and pop place I generally know what I am getting and dissect it into its ingredients like a quesadilla or pasta.

I eat out a lot, because I have weekly meetups with friends. Definitely looking ahead of time is good. I've found after a few months of that, I can pretty much look at a menu and pick out the things I could reasonably eat and I know the pitfalls - the bread or chips a lot of places bring you, brownie sundaes which are a while day's worth of calories, etc.

The really hard thing is eating at people's homes, especially when it's a unique or authentically ethnic recipe, but even that can be worked around.