How do you deal with feeling fatigued due to allergies?

Several ways. Allergies certainly tire a person, and many antihistamines (the usual treatment) make one sleepy as well. I would use one of the newer antihistamines that do not get into the brain, like Claritin or allegra, to reduce the severity of symptoms. Clearing the nasal passages helps reduce the effort of breathing and helps with air exchange so a decongestant and nasal saline rinses can also help.
Other thoughts. Sometimes a food sensitivity can make you tired. Pay attention to what you eat. Fatigue is just a symptom of something deeper. Make sure you are not anemic or depressed as these problems also cause fatigue.