Plan out your reasonable portion ahead of time, before you feel hungry and before your emotions can take over. And once it's time to eat,
stick to your decision. Don't allow yourself any other options, don't justify eating more by telling yourself "maybe just this once," and consider how your decision will make you feel once you've completed your meal. How will you feel an hour later? What about the next day? Will looking back a week from now leave you feeling satisfied or will you regret how your decisions are affecting your health?
Does anyone ever say, "Oh, my jeans are way too tight, but all the overeating I've been doing is worth it!"
Not saying that one should never have treats, but we all have temptations that we have to learn to deal with and we have to know when to be able to say yes or no. I've been doing the above for so long now that it's almost become automatic.