You should look in a mirror, and say things, like "Oooo", "aaah", "Eeeeeeeee", "Ehhhh" and so on, and see how your mouth shapes differently for every sound!
Also, the mouth typically changes from syllable to syllable, like "Hey" is one syllable, and only needs one motion. But "Sexy" has two syllables, and three sounds; "Sex-/-xy"(choose where you want the x to be :'3) and "Se", "x" and "y". You could draw a sound-map of how the different sounds look while you're talking, or you could find something on tha awesome intarwebs ^w^
..something like this. (But make sure you find something better than this one, though..) [link]