One reason untrained singers sound untrained is when vowels go splat.
Splatting is when the vowel spreads out and flattens, like a paintball hitting a wall.
This is a huge topic, but I’ll boil it down into three words:
Vertical. Not horizontal.
Click here to hear a vertical ‘ee’, where the lips are stretched into a smile.
Click here to hear a horizontal ‘ee’ where the lips are making a vertical ‘oo’ shape.
Which do you like better?
(c) 2009 Adrienne Osborn