There some very good vocal exercises entailed in different warm ups or daily routines that work on the mixed voice. Gavin, To keep it simple so you don't go into information overlaod.Think of mixed voice as a combination of the head voice,typically you hear sopranos sing in this, it is"light and airy" and chest voice is"the voice you use in everyday communication" Commonly, men will pull too much chest voice into high notes making it sound flat or as if you are yelling. You'll find different exercises here, like the Myaah exercise that work Mixed voice. I use an exericise utilizing the word" Goog". It is great for finding your mixed voice register which is your 3rd register. Be patient! Good things take time.