Not knowing what you will eat tomorrow will throw you into the 'grab what you have' panic when you start to feel hungry. Preparing a meal on the fly makes you highly likely to choose higher-calorie, less nutritious food. So, when you have lunch or dinner today, make the plan for the next day's meals. It's much easier if you do this when you are not hungry and it gives you enough time to chop veggies ahead of time, take something out of the freezer to defrost or do part of the cooking today.