How to speak clearly and stop sounding nasally?

I tend to mumble when i speak and i sound nasally. Weirdly though i can here my self clearly enough but people seem to hear me differently from what i think i sound like. On recordings of my voice though it sounds different and i presume that that's what i sound like to other people. This may sound strange but i can't seem to blow my nose. Obviously i can blow air out of it but nothing seems to come out apart from air no matter how hard i blow. Apart from the obvious "open your mouth wider" thing is there any way to help my stop mumbling and sound nasally?