three things that will prob help

1. pretend you are sucking air in through a straw. notice the way it feels going in.   it does down way deep, to your diaphragm. now sing, letting it go out the same way

2. pretend you put very hot food in your mouth and are too polite to spit it out. you instinctively open the back of your mouth to cool if off. now try singing with the back of your mouth still open that way. you will have to close it up to make consonants, but it will still help

3. if you are sitting on a folding chair, scoot forward to the front edge of it so your body is not jack-knifed shut