Vowels are the music of our speech; they carry the tone and must be pronounced like the open and pure letters they are. Consonants are the bones of speech; they must be sharp and precise. Diphthongs are a union of two vowels that form one syllable, but that one syllable must be formed properly.
When forming all vowels, the tip of your tongue should be behind your lower teeth. The two major problems people have with vowels are elongating the short ones and not drawing out the long ones. Keep your jaw active and your tongue flexible and you will avoid these mistakes.
Practice: Explore the differences between "he bit the dust," where the vowels are short and clipped, and "the long sleeve," where the sounds should be drawn out.
Reading out loud is still the best way to become aware of the various sounds speakers face. Inside our small, 26-letter alphabet lurk a surprising variety of combinations. We have only five vowels, but 14 vowel sounds are possible. Our 19 consonants can produce 25 consonant sounds—like the "dg" in udge. The more aware you are of these components, the more practiced you are at uttering them, the more polished you will sound.