Actually, it might make more sense to try to figure out why it works some times and not others, which can even happen in the same person... like, uh, me. If I'm careful about what I keep in the house, I wind up gorging on anything tasty almost as soon as I get it, and buying less healthy 'treats' more often ('just this once', repeatedly) when I go out. If I keep goodies at home, I'll ignore them for a while, but then decide something along the lines of "it'd be a shame to let this go to waste" and eat them anyway.

There are different mental states involved in each of those situations, but I don't know what triggers the switch from one to another.