I've been off and on as well. Often I know that it's not worth it to cheat, but I do it anyway. I'm getting better using this specific technique:

Mostly I get by through the use of visualization combined with motivational comments. I'll tell you what I mean in a minute...

I believe that I cheat when I go into a semi-hypnotic state. In other words, when pizza (for example) is in front of me, I allow myself to forget all of the negative consequences of bad eating. I allow myself to focus on the smell and the taste of the food. Bad move...

Now here's how I get by:

1. I visualize myself as fat, as unhappy, as ugly, and depressed.

This gives me a reminder of why pizza, in the grand scheme of things, is not worth it. Don't stay in this mode too long.

2. I visualize myself as healthy, lean, attractive, and happy.

Then I focus on this even more.

I make the concious decision in my mind: "I will not eat this pizza. I don't eat that many carbs. Let them eat that poison if they want. I know that I am losing weight by saying no."

Afterwards, I feel much better. I eat something sensible and the cycle continues.

Also, don't let yourself get tricked into "I'll start on Monday," or "I've already cheated today, so I'll cheat again, and start tomorrow." If you do this every day, you'll never start the diet.

In summary, STOP yourself before digging into the food pile. THINK consiously, deliberately, and with passion. Give yourself at least 2 minutes of good, hard decision making before even getting near the food.

Hope this helps. Good luck.