It takes about 20 minutes for your body to know you're not hungry anymore after eating... eat slowly to give your body time to process what's happening, that it's being feed.
One of the things that bothers me the most is when people talk with their mouth full. I was raised not to talk with food in my mouth, funny thing is I'm constantly having to tell my mother (and daughter) the same thing... "Finish the food in your mouth, then tell me".
We rush through our meals, always trying to eat and do something else at the same time - like talking... if we didn't talk with our mouths full it would slow our eating down and help give our body time to know it's getting full.
So next time you're out for a meal try an experiment... chew your food, don't talk with food in your mouth, and enjoy the company you're with and the food you're eating... I bet you'll probably eat less.