Man, I'm all over these boards today.
A few suggestions for restaurants:
If you know where you're going ahead of time, do your research. A lot of places are offering lighter options and many have their nutritional information available online. And if they don't, try a website like
http://www.calorieking.com/foods/.
Don't get fooled by salads. Seriously, some of the salads are so loaded with cheese and dressing they're not worth eating.
Eat something before you go. Even if it's just a tiny snack (like a few almonds or some veggies) it'll take the edge off your hunger and make sure you don't order more food than you need.
Drink water. This one's pretty obvious. And drink LOTS of it with your meal.
It might also be a good idea to let your friends know if they make a choice that doesn't work for you. For example, before I started dieting I was a big fan of the local Chinese buffet. Not anymore. And now when my roommate wants to go there, I have to remind her that I can't. No big deal.
Good luck!