First is to make sure you are sleeping. Sleep is a natural process that restores the mind and body, and a lack of sleep will often impair one's ability to perform both mentally and physically. Researchers suggest that most adults require 7 to 8 hours of sleep, and have also found that we can't "catch" up on our sleep. Research also points to a higher mortality rate in adults that get less than 7 hours of sleep a night. So make sure you're aiming for at least 7 hours of sleep a night, and if you still feel tired the next day, you may need even more.
Second is to make sure you are eating well throughout the day, so make sure you eat breakfast, and try to eat 4 to 6 small meals frequently throughout the day. You have to watch out for simple carbohydrates such as sugary snacks. Often you'll have a surge of energy, that will soon disappear. If you look at each time you eat as a small meal instead of a snack, you're more likely to choose something healthier. Each small meal should have carbohydrates, preferably complex carbohydrates like whole grains, protein, and high quality fats like those found in fish or nuts. A good example for a quick small meal is yogurt with real fruit a granola. This will help with your energy until the next time you eat. If you have significant time restraints, or always eat 'on the go', then carry a few healthy snack bars with you, with the same requirements of your small meal: complex carbohydrates, protein, and high quality fats. So avoid the drive through for a quick fix.
You also want to pay attention to what you're drinking. You shouldn't drink more than two to three cups of coffee in the morning, and avoid drinking caffeinated beverages in the afternoon as that can also interfere with your sleep. Make sure you're getting at least 6 to 8 glasses of water every day, as mild dehydration, especially in the summer heat can easily lead to fatigue.
Try to insert some exercise into your schedule at least every other day if not daily. The department of Health and Human Services recommends two and a half hours of exercise weekly. If that sounds like a daunting task, start out with ten minutes of walking or even stepping in place. If you do that everyday for a week, that's halfway there. If you find it difficult to exercise for 30 to 40 minutes at a time, you can always break that up into 10 or 15 minute intervals. Making this a habit will give you much more energy throughout the day.