Eat less at lunch. Your body has to work hard to metabolize a heavy lunch full of carbohydrates and sugars, leading to that mid-afternoon slump. A lighter lunch will keep you charged up and ready to take on the rest of the day. Try soup and a sandwich with a bowl of rice with oil less curry. Eat chicken, turkey or fish as your protein, and fill up on fruits and vegetables instead of French fries or pasta.
Don't smoke. The chemicals in cigarettes can cause profound fatigue, and the lack of clean air in your lungs can make your body work harder to stay oxygenated. Stopping smoking will make you feel more energetic all day long.