Wish I had more resources to help with this.
To relax the neck, there are several things one can try.
Actively tighten the posterior neck muscles, by isometrically "trying" to lift your chin to the ceiling but not allowing it to move upward. Then relax those same muscles you were just trying to contract. It is likely that the back of the head will try to move backwards and up (ie. retract and chin will move down). Eventually you will get to the point where no pre-contraction will be necessary.
Or you can do the "string pulling up on the top of your head" visualization, relaxing the neck muscles so that your cervical spine can elongate. This one can be tougher to do. Make sure your are not *actively* trying to elongate your neck with muscular energy. I made that mistake for a long while.
A single session with a good Z-Health instructor should easily get you to where you need to be with this technique, that might be worth all the money spent right there.
Found this neat little article on relaxing for guitar playing, thought there might be a pearl or two that could be used here: http://www.theguitarsuite.com/relaxing.html
Relaxing the face muscles (eyebrows, cheeks) can be a great warm-up leading into getting the neck muscles relaxed.
If you are scrunching your face at all, then I guarantee you your neck muscles are NOT relaxed.
I hope that's enough to get you started. This is a worthwhile thing to pursue.