You have to mentally prepare yourself in advance. When ever I have been sucessful at saying no I have been in a weight loss frame of mind. Plan on eating no more than 2000 calories a day and do it using a food journal.
I also wait 30 minutes after eating before making any decisions about eating additional food by that time I usually dont want any extra or can make a healthy choice.
Drinking adequate amts. of water each day is also critical.