Keeping the microphone "off-axis" or at an angle to the speaker is crucial for many microphone to avoid plosives: the popping Ps and hard consonant Cs and Ks.
Place the microphone roughly at 45 degrees, between the ear and the nose.
The microphone can be below, at, or above the mouth as long as the speaker does not turn directly toward the microphone as a reflexive motion (which is why it's usually best to place it down and out of the line of sight with the reporter).
Pay attention to the speaker's movements, be agile and compensate on-the-fly, moving the microphone as needed.
For non-powered/dynamic microphones such as those used in most rough and tumble radio work, a distance equivalent to the width of a closed fist is generally a good starting point, pulling back only as much as needed to make the subject comfortable but not much more than two times that distance in order to stay "on microphone".
For powered/condenser microphones, such as television shotguns, distance is far more flexible although placement is still an issue as these microphones are far more sensitive.