The proper placement of the microphone in front of your mouth is crucial on headsets. The microphone element is very directional and is sensitive to the distance that it is placed between your lips and the microphone. If placed too far away or angled you may not be able to be heard clearly.
Be certain that your aircraft communications system volume control is able to be accessed with ease. This control affects the strength of the communications signal coming into the headset even when the ANR is turned to off.
Try on all headsets before purchasing to be sure they properly fit your head and are comfortable.
People who wear glasses may want to consider liquid filled seals because the liquid conforms around the temple bars better than the foam filled seals.
Do not set the volume controls at a level that is high enough to impair your hearing during extended periods of headset use.
Be sure the ear cups are kept free of debris.
Hook headsets up to an intercom to hear the sound quality of the set.