1. Follow the balanced plate rule
It is recommended that meals should comprise of 50% fruits and vegetables, 25% protein and 25% carbohydrate so it is important to bear this in mind when putting together a lunch box.
You can get hold of some great lunch boxes with different compartments which help you to portion out the different food groups.
Taking this concept very simply it might look like - 1 portion of fruit (apple), 1 portion of vegetable (carrot sticks), 1 portion protein and 1 of carbohydrate (chicken in a wrap with salad and low-fat mayo)