Let me share with you how I got my boy to sit more and engage that hind end... TRANSITIONS!!!!!!! A billion. Carry sugar with you. You can do so many exercises with transitions figure eights, spiraling in and out, transitions within the gait that your in, asking for more power and then bringing it back to dancing steps to pushing out with power again.
The other thing to remember, the headset will come on its own when the horse is ridden correctly, straight with power and relaxed rider. After the horse and rider have formed a pattern in the past for tention and bracing its a long road to over come more mentally then ever.
I just had a lesson on getting Oliver to SIT more and lift his front end. The relization I came too in the lesson is that you have to let go, let them go forward and it will feel so scary at first up by us holing on and constiantly fidling with the face for what you call a head set all we are doing is restricting the aid to go forward with your seat and leg.
So let the head be up, when it goes up just add insde leg to your STEADY outside contact. Doing this through circles going from walk to trot, spiriling the circle in and out to get the horse listening to that inside leg and going into that outside contact is what you want.
Now another thing for transitions. Hold your horse responsible to continue through the transition. I suggest using your voice and if you carry a dressage whip, tap the horse up and in the trantion. You should think of ALL transitions, especially the downward ones as upward, your sitting the hi end to lighten and lift the front end up.
Now remeber the "fastest" way to get through this phase in training is praise. Work on this for 10 minuets and really hold the horse responsible for doing work. Then make a huge fuss and give a reward. Then work on somthing else and come back to it.
Also know you can you do this anywhere. In hand, on the trails in the arena, in the pasture (should it be safe)
Keep in mind that when you really start getting this down your horses back muscles will be sore. this is a new way of traveling for him/her and takes a lot of back muscle so lots of walking and don't over do it. Short work periods on this with lots of walk breaks on a long rein for him/her to stretch down.
Its a lot of hard for but I would say if you stick to the exercises and really commit to riding this way for the next couple weeks in 2-3 weeks you should feel a big difference!!
Here is a video that you can see the difference of the horse dying and quitting in the transitions down and then getting the horse to keep fallowing through with that hind leg all through the transition.
Hope this helps. If you need any further help or want any video footage watched let me know!
Stick with it!!!