Eat more often. Take frequent breaks and have some fruit. Take a real break for lunch. Your body needs nourishment to prevent heat stroke and keep your mind sharp. Drink plenty of fluids.
Water is best, but fruit juice makes a nice change of pace.
Caffeine can aggravate heat stress in the summer. Get plenty of sleep. Your body needs to recover from the physical demands put on it during the application season. You'll have fewer pesticide spills when you are alert. Count to 10.