First of all, remember that while affectation is odious, crudeness must be overcome. A low voice is always pleasing, not whispered or murmured, but low in pitch. Do not talk at the top of your head, nor at the top of your lungs. Do not slur whole sentences together; on the other hand, do not pronounce as though each syllable were a separate tongue and lip exercise.
As a nation we do not talk so much too fast, as too loud. Tens of thousands twang and slur and shout and burr! Many of us drawl and many others of us race tongues and breath at full speed, but, as already said, the speed of our speech does not matter so much. Pitch of voice matters very much and so does pronunciation—enunciation is not so essential—except to one who speaks in public.
Enunciation means the articulation of whatever you have to say distinctly and clearly. Pronunciation is the proper sounding of consonants, vowels and the accentuation of each syllable.
There is no better way to cultivate a perfect pronunciation; apart from association with cultivated people, than by getting a small pronouncing dictionary of words in ordinary use, and reading it word by word, marking and studying any that you use frequently and mispronounce. When you know them, then read any book at random slowly aloud to yourself, very carefully pronouncing each word. The consciousness of this exercise may make you stilted in conversation at first, but by and by the "sense" or "impulse" to speak correctly will come.
This is a method that has been followed by many men handicapped in youth through lack of education, who have become prominent in public life, and by many women, who likewise handicapped by circumstances, have not only made possible a creditable position for themselves, but have then given their children the inestimable advantage of learning their mother tongue correctly at their mother's knee.