If you're the kind of singer who doesn't move your lips much no matter what the words are, I have a few tips which will make your diction clearer and your output louder, all without sounding like you're standing on a musical theater stage.
Think of vowels as uninterrupted flows of resonated air. When your vocal cords vibrate, they make a complex buzzy sound, not unlike a duck call used by hunters. Then you shape your mouth and throat to amplify certain of those harmonics generated by your vocal cords to achieve different vowels like ah, oh, oo and so on. If you sing oh, then gradually change your mouth shape to ee or oo, you may be able to detect the shift in harmonics.
I like to break down vowels into three basic categoris: Smileys, Fishes and Puckers. Smileys look like you're smiling; stick your lips out for the fishes and puckers are in between smileys and fishes, rather like you're sucking on a lemon with a slight puckering on the sides of your mouth.
Then let's further divide these three categories into sizes small, medium and large. These sizes refer to how many fingers you can fit in between your upper and lower teeth.
For example, a small smiley is the ee vowel, a medium smiley is the ay vowel and the large smiley is the ah vowel. The small size is a one-finger mouth, the medium size is a two-finger mouth and the large is a three-finger mouth (unless you have fat fingers!).
The Fishes in order are: oo - small, oh - medium, aw - large. There are five Puckers: ih - small, eh and U (like in the words good and could) are mediums and the two large Puckers are aa (as in cat) and uh (as in love). Play with the Smileys, Fishes and Puckers to get the most out of your vowels