How to stop mumbling so much?!?

Basically, you just want to open your mouth more when you talk. Practice reciting things at home with over-enunciated syllables, and opening your mouth more than you normally would. Two finger spaces should suffice. Getting something exactly where you want it to be is like stretching an elastic band, to loosen it up, you have to stretch it more than you need to. I don't know if that made sense :P but basically, it will feel ridiculous at first, but with enough practice, it'll become second nature.