For those times when you forget to pack food, or something unexpected comes up and you can't get to anywhere with healthy food choices – have some snacks with you at all times.
Foods that stay fresh for long periods of time and do not refrigeration are key. Pack them in your car's glove compartment, purse, gym bag, etc.
Some examples are; Nuts, beef jerky, dried fruits, protein bars, whey powder (just add water). Maybe they aren't all in your diet plan but they can be a better choice than some other alternatives when you are stuck!