You can only play 'by ear' using musical elements that you're familiar with. So you need to get familiar with LOTS of music, in LOTS of styles - what it sounds like, and what your fingers have to do to get that sound.
Sorry, @Tim, but by far the most efficient way to do this is by reading lots of music. Not necessarily by perfecting a small number of difficult pieces (though you need technique too.)
Develop your 'by ear' playing skills AND your sightreading skills. They will nurture each other. Remember, the aim is to play the piece. If the printed music is available, it's foolish to reject that route!