Eat smaller, more frequent meals.
Snack throughout the day. By eating smaller but more frequent “meals”, you will maintain a steady dose of energy instead of experiencing food comas. Don’t snack on fatty and sugar laden junk food though. You may get a short 30 minute burst of hyper-alertness but it’ll be quickly followed by a debilitating crash.
These are two habits that should go together. When you eat a meal, try to make it small enough that it satisfies your hunger but doesn't make you feel full. Then in 3-4 hours when you feel hungry again, eat some more. Having 5 smallish meals and snacks each day instead of having 3, or more typically, 2 big meals a day helps speed up your metabolism to process and burn calories more quickly. Think about it this way: Sumo wrestlers fatten themselves up by eating only one HUGE meal a day and then going to sleep right afterwards.