I copy this part from my diary, because really improved the travers for me and Mucki:
I found a very helpful exercise, which helped us immensely in getting the hang of the travers. It's called "Schlängeln" in German and here is a video of it
(with German description, but it's actually self-explaining...).
It's basically a series of shoulder-ins on a zig-zag line, while I am walking backwards myself on a (more or less) straight line. The most important part is the constant lifting of the shoulder, change of flexion and tilt of the pelvis. It makes the horse very supple and light and puts him basically in the right position for the travers, only makes it easier at first by keeping the zig-zag path.
Later, I just hint at the zig-zag movement - I ask for the bend and then for just forward movement and so get the travers. Whenever I start to lose the flexion in the neck, I ask for the diagonal/lateral movement again and Mucki falls into his well trained shoulder-in mode again with the correct flexion.