Here is one really great exercise from the Book What Are You Hungry For? By Deepak Chopra. It is a wonderful way to become more self-aware, and self-awareness is the key to breaking any bad habit whether it be overeating, smoking or excessive drinking.
S-Stop what you are doing
T-Take one minute to breath consciously counting from 1-20 on each inhale and exhale. For example exhale on 1 and exhale on 2, inhale 3, exhale 4 and so on.
O-Observe your body. How hungry are you from a scale of 1-5. Five being famished and 1 being not hungry at all.
P-Proceed with awareness.