It helps me to menu plan at least two to three days ahead, but I have done a whole week. And by that, I mean plugging it all (even snacks) into the daily thing on here. Of course I can make changes (especially for snacks - may not know what I'm wanting), but it really helps me to stick to the plan.
Also, I have to be diligent on measuring or I slowly tend to creep up in portion size.