Great things mentioned, Puck.
I believe on that second picture someone even used a red line to show how the line from the riders hand to the bit is not straight, but V shaped.
On top of all of this, the idea of constant, consistent contact must be established. Start slow and work your way up, beginning with a strong inside leg to outside rein feel. I'd be willing to bet every time that girl on the right posts, she moves her hands up and down, throwing away the contact then finding it again which leads to a jerk on the horses mouth every stride.
Start by getting a nice, swinging, forward walk from your boy and just focus on relaxation and taking a feel of the reins. You want to be able to feel the horse's mouth in your hands, but not roughly. I like to think of it as two people holding hands- you aren't squeezing and jerking on your friends wrist, but you are quietly there and feeling them.
I also like to work on inside and outside bending, in and out spirals, and square corners to really get in tune with that outside rein and stretch down and into the bending.
Getting an instructor out every once in a while to help you put these concepts in motion is great too. Once your horse realizes how good it feels to stretch down over his topline, he will be so much happier and more willing to do all these things for you.
We are not human beings having a spiritual experience. We are spiritual beings having a human experience. -Pierre Teilhard de Chardin