Answered[ Edited ]
pocketvenus is totally right, over shampooing both kills your hair shafts (especially if you have longer hair) and causes the sebaceous glands in your hair root to secrete more. You might need to change shampoos to find the one that irritates you're scalp the least.
In the mean time, my advice to you is to shampoo less frequently to let your glands re-adapt, get some dry shampoo for quick fixes on the days you're skipping out (BTW Oscar Blandi makes them in handy purse sized formats). Also, Try to use less shampoo each time you wash and try to focus it to just your crown/scalp region. The opposite goes for conditioners, try to limit that to the bottom part of your hair, starting at the level of your ears.