If I am full when I'm at the grocery store I don't get anything that's not on my list. Also I stick to the "edges" of the store: produce, butter, egg and meat departments. I no longer buy anything with preservatives or long lists of ingredients. If it's not in the house, I don't eat it!
Being on a budget helps wonders as well. I only have a certain number of dollars to spend on food, which really helps me not browse when I'm at the store. I can only afford to get what I came for. I aim for the highest nutritional value possible, so down the road I won't have high medical bills.