This part seems obvious, but many speeches are ruined simply because the person making the speech doesn’t know how to hold the mic.
See the diagram. Gary’s got that mic nice and close to his mouth. As a general rule, keep the mic around 2-6 inches from your mouth. Direct the microphone grill towards your mouth, but slightly off-center.
If you hold the mic too far below your mouth, or move it around while you speak, the audience will not be able to hear you. This is due to poor mic technique, there is very little a sound person can do to improve the situation when the user has poor mic technique.