Leo Babauta offers the secret rule of changing anything:
When we have urges to eat something we know is bad for us, we often give in. But is it that simple? The truth is that our mind is actually rationalizing why we should just eat that cake, why it’s too hard to not eat it, why it isn’t that bad to eat it. It asks why we’re putting ourselves through pain, why can’t we let ourselves just live, and don’t we deserve that treat?
All of this happens without our noticing, usually. It’s quiet, in the background of our consciousness, but it’s there. And it’s incredibly powerful. It’s even more powerful when we’re not aware it’s happening.