Causes of Flat Beer
One of the primary causes of flat beer is basic: the glass. Beer glasses should be kept for beer only. Using the same glass for beer as for chocolate milk is not a good idea. Wash and rinse beer glasses in an area that is separate from where you wash dishes that have been exposed to food. Hand-wash and rinse thoroughly with very hot water. 
Allow to air dry – lint from toweling can kill a head. One more thing – if you are eating cheeses, burgers, fries or pizza with your brew, expect the head to disappear! Grease is a head killer!
Flat beer could also be due to malfunction of storage or dispensing equipment. Holding a beer glass too close to the faucet is a definite no-no. When tapping a keg, always connect and turn on the gas supply before connecting the beer faucet. If the connection is not completed in this synchronized manner, some of the internal carbonation will be dissipated while moving the beer into the beer line. Once connected, the CO2 should never be shut off.
Serving beer too cold not only kills the rocky foam, but also kills your taste-buds, so watch the temperature of your brew.