I firmly believe that aural comprehension - comprehension of pitch and rhythm by ear - is the foundation of musical development and healthy musical life.
A good ear is just as attainable as a good instrumental technique. It makes for more pleasure in music, more understanding and better listening and playing. Playing by ear and sight-singing become easy, and the superficial mysteries of music are solved, revealing deeper ones.
Every student hears differently, so one-to-one tuition is ideal. I teach from Modus Vetus and Modus Novus by Lars Edlund, texts in use at the ANU, as well as providing many exercises of my own.