Hi there,
i can give u couple of pointers and maybe it might a good idea to just get a few leasons with a voice teacher if iron out ur problem. Generally choir singing is great for training in tunes and harmony although it doesn't alway teach specific technique. So my advice would be to keep practicing a 5up 4down note scale of ay-ee-ah-oh-oo (up)ay-ee-ah-h (down), also imagen there is a pencil between ur teeth at the back of ur mouth when u sing so ur teeth never touch but make sure there is not tension in ur jaw, maybe pretend to chew gum in an exaggerated manner before u sing to relax and stretch the muscles.
once thing, if u emerce urself in the emotion and mood of a song i've noticed good diction tends to follow as there is more invested and the body tends to follow the mind in making a bigger effort.
Hope i've helpped..Anna

Hi James,

Try the Diction page
http://www.vocalist.org.uk/diction.html

Hope this helps
Tracey
Vocalist Webmistress