I'd modify Leslie's advice to say when you feel like you're crashing, have a protein (cheese, pb, nuts, as mentioned) AND a bit of a carb (crackers, fruit, piece of toast). Nothing extravagant, nothing huge. You need both to really get your energy level up if it's blood-sugar related tiredness. The carbs give you energy, while the protein slows the absorption of the carbs into your body to give you level, consistent energy, not a giant spike followed by an awful, tired, crabby low.
Trying that when you feel tired for a week or so will at least tell you if your energy level is related to what you eat, how often you eat, and when you eat.
A multivitamin is another good suggestion.... Especially B vitamins or Iron (deficiencies in both cause lethargy).
Being tired all the time is NOT normal, and if the problem persists, I'd say you ought to go to a doctor. That's how I found out I was hypoglycemic.
Hope that helps
