1. Always serve beer out of a clean glass- dirt and debris in a glass can cause CO2 to abruptly come out of solution, detracting from the beer's flavor. Soaps and residue from detergents, lint from a towel used to dry off the glass can eliminate the head of the beer. Using a correct detergent, allowing glassware to air-dry and using a glass rinser before pouring a beer will provide you with the best results.
2. Never serve beer in a frosted mug- A frosted glass not only mutes the flavor of the beer, but the cold glass also releases much of the CO2 when the beer touches it so you've just flattened the beer.
Clean glass on the left with a proper head. Dirty glass on the right quickly loses it's head & aroma