Good pronunciation begins with good listening. Good listening begins with an understanding of how sounds are produced.
By learning how the tongue, the lips, the voice, the breath, etc. can be manipulated to produce different sounds, one begins to then recognize subtle variations in pronunciation from one person to the next. He can then incorporate these differences into his own speech more quickly and easily, because he already has notions of how the sounds are produced.
Jeremy Coffman, English teacher & diction coach, Paris