When i travel by plane and get severe headaches and i am assuming is due to my allergies. what can i do to prevent these headaches?

Do they throb?: Most planes are pressurized at 8000 ft altitude and the air is quite dry. Thus you may suffer from high altitude sickness (rare but not impossible at 8000 ft) if you have migraines, time zone changes often trigger migraines. Since you do have nasal congestion, the first thing to do is to see your doctor to treat the congestion and see if this solves the problem. Drink enough water also.

Decongestants: Decongestants will help if your sinuses are clogged. Be aware that decongestants can cause an increase in your blood pressure and other side effects so check with your doctor to see if it is ok to take them. Decongestant nasal sprays are also helpful but do not use them for more than 2 -3 consecutive days.