DressageArena.org

THE BAD NEWS:
You are going to hate me for this, but there’s no easy way to get it. Conventional methods, like see-sawing, end up with a pulling, bracey horse who’s in a lot of pain. Seen the blue tongue video yet?
http://www.dressagedisgrace.com/uncut-blue-tongue-video
This is taking the "see-saw tug and force" method to the extreme. Forcing a headset only makes the horse resistant and less manouverable.

There is no "cue" for a dressage headset, and TBH, I have no idea why you would need a crop.

THE GOOD NEWS:
You can begin getting a nice outline working over time, sometimes even a few rides depending on your horse’s fitness.

Working "on the bit" begins with the horse stepping through its haunches. The rider should keep an elastic contact on the reins while maintaining the horse’s impulsion. To relax the jaw, go on a 20 meter circle, take your inside hand to your hip, and gently vibrate the rein or turn it like you’d turn a key in a lock. Nothing huge is going to happen, but you will feel the horse give and most likely stretch down. You have the beginning of "on the bit" right there.

At this point, you’ll probably have a nice, schwungy trot. If you don’t, you won’t get anywhere- you always work on the horse’s haunches and back before you even think about the neck. Until your horse is moving through, all you can do is keep a good contact and encourage relaxation and impulsion.

A lot of trainers use the see-saw method. Any trainer worth their salt will allow you to pursue classical methods, as long as you and your horse are being safe, of course- if they do not, it’s time to go shopping for a new trainer.

Why are you using a crop to get a headset?

I looked at pictures and saw the beautiful headset! I’ve been working on that with my horse. What I do is squeeze her and bump the reins(jerk each rein back in an alternating pattern) and releasing when she brings her head down. I need be, I bump really hard until she gets the message. If she pops it back up, I bump again no matter what I am doing. For awhile, she too was ignoring me. I bought a pair of side-reins and lunged her in them. It did the trick.

What cue are you using?

EDIT: Sorry for my mistake. I thought you got the headset all the same way. Whoops! You learn something new everyday.

With dressage you don’t bump your horses mouth to ask for a headset. You need to concentrate on getting her butt under her and the front end will lift and the head will correct itself. Hunters/jumpers bump the mouth asking for head. The whole point is not the headset its the frame the horse is carrying itself around the arena in. You want a nice round back and a tucked butt. Concentrate on small, but not tiny circles, maybe 10 meters. Keep a nice steady bend but not so much that her head hangs in the middle of the circle allowing her to disengage her hindquarters. Dont forget about your outside rein once you have the bend. Hold her and use your legs to drive her into the bit. This will help with her extensions at the trot and give you a rocking horse type canter. I agree with lunging with side reins, but ask your trainer to help you get the proper tension in them.

Get a good book on dressage riding and training…there are hundreds of them out there.
My answer to this question would be a novel in itself!
I will give a few basics though…
It seems a lot of people, including trainers, not implicating yours but there are trainers, want a quick fix and force the horse into a "frame" or dressage headset. This is unnatural for the horse, and yes, they will resist. You must look at the horse as a whole, he/she needs to be carrying itself properly first, engaging all the correct muscles. The horse should be "under you" and by that I mean getting the back legs working harder so the back stretches and the neck drops and begins to form an arch.
When the horse is engaging the correct muscles, it will seem as if you are sitting higher.
The horse will be extremly receptive to transitions, be more flexible. You will get so much more out of your horse when it is using itself properly.
First things first, build the topline…lots of strength training for the horse.
Training on the ground with side reins helps, it does build muscle and help the horse to hold itself in the correct position.
When introducing this type of riding to the horse you should ride the horse "long and low" for a while before asking it to raise it’s head into the traditional "frame".
Look for a good book and educate yourself on what the top dressage riders do when riding their horses. It takes years to make a horse really good at this.