If you're going to snack, tell yourself you can, but only in front of a mirror while watching yourself. This saved me a couple of times from eating when I knew I shouldn't.

Just the thought of having to watch myself eat food I didn't need made me stop.

Also, filling up on water works. Another trick is to pinch your nose shut if you're snacking. You can't taste anything at all and the satisfaction is just gone. You can't even taste pure cinnamon with your nose pinched shut, it's just a gritty powder.