Microphones are common technology and yet few people understand how they work. For our purposes here, there are unidirectional microphones and omnidirectional microphones.

A unidirectional microphone only records the sound from one direction. If it's pointed at the mouth of the speaker it won't record noise from the audience which makes the resulting recording better.

If it's however pointed at the ceiling while it's in front of the mouth of the speaker it will do a poor job at recording the speaker. 

This leads to the first rule of microphone usage:
Hold the microphone pointing towards your mouth.

Microphones react to sound waves and sound waves are movement of air. If you exhale into a microphone, the microphone will record the exhale. Given Newtons laws, the exhale isn't omnidirectional either but the air that comes out of your mouth from the exhale has a clear direction. If you hold the microphone in front of your mouth, it will get hit by the air. 

Our second rule of microphone usage is:

Keep the microphone to the right side of your mouth if you hold it in your right hand and correspondingly on the left side of your mouth if you hold it with your left hand.

Let combine the two rules into rule zero:

Hold the microphone to the side of your mouth in a way that points towards your mouth.