Eat breakfast.
Eating a healthy breakfast fills you up with energy for the day and also decreases your desire to eat a big lunch in the middle of the day.
Not only that, breakfast can be very healthy, quick, and inexpensive.
A bowl of oatmeal in the morning is often the one thing that keeps me from running out to eat an expensive lunch later in the day – and it keeps me peppy and full of energy for the entire morning instead of in a coffee-laced daze.