Try practicing your vowels in front of a mirror to ensure that you develop the habit of using the correct mouth shapes.
As in "Meet": Do not allow your mouth to spread sideways, as this will produce extra tension. Your tongue should be resting comfortably on the back of your bottom teeth, but will arch slightly forward.
As in "Say": This vowel is sometimes pronounced as a diphthong, because the vowel starts with one sound and ends with another (EH-EE). Keep the tongue resting on the bottom of the mouth and the jaw relaxed.
As in the expression "Ah": Try not to force this vowel. The sound should come from low in your throat. This is an excellent vowel to use when practicing scales and exercises, as it allows for a very open relaxed pathway and doesn’t put tension on the larynx.
As in "Coat": Allow the jaw to drop naturally and keep the mouth relaxed and in a similar position to the AH vowel. The lips should move forward and the inside of the mouth should feel open.
As in "Room": The lips should form an oval shape. Once again, the jaw should be relaxed and the tongue should sit comfortably on the bottom of the mouth.