From Vasant Lad's Yoga Journal Post: Unusual mental or physical exertion, stress, and lack of sleep can make people tired. Prevention in these cases simply requires self-observation. Sometimes it means not pushing the body and mind beyond its limits. Other times it means walking or doing some physical work to help increase the body's energy level.
Other major causes of fatigue include anemia, low gastric fire, and weakness of the liver. If the fatigue is due to anemia, eating iron-rich foods and blood builders such as pomegranate juice, grapes or grape juice, and beets or beet/carrot juice, can help. Right nostril breathing (Surya Pranayama) is also helpful, as it stimulates the liver, which plays an important role in building the blood. For fatigue caused by physical exertion, drinking fresh orange juice with a pinch of rock salt gives a quick boost. Add 10 drops of lime juice to help the body cool down.
When agni (the digestive fire) is low, digestion will be sluggish, bringing the energy level down. One way to raise agni is to chop or grate a little fresh ginger, add a few drops of lime juice and a pinch of salt, and chew it before meals. Also, it is best to avoid cold or iced drinks, as they counteract agni and impede effective digestion. Instead, take small sips of warm water while eating.