Don't lock the knees. The line should cross over the apex of your skull, which can be a little tender.  As your head moves forward, you should feel it lift up and move back (as opposed to down and forward), pulling your shoulders back and your chest up.  Remember this position as proper posture.
Stand erect, feet hip-width apart and parallel. Imagine a line connecting the tops of your ears over the top of you head. Tilt your head back. Imagine a string pulling straight up from the apex of your skull.