Here's something that for some crazy reason is not well known or frequently used, which is really pretty stupid, because it's simple to use and it works correctly. The chambon is the only piece of equipment I'm aware of that encourages and rewards stretching forward and down. Side reins, draw reins, the so-called "neck stretcher", they all work to pull the horse's head back, or at best do nothing (so why bother using them?)
First, let's look at River longeing in a chambon:
How about this?
Red is a strap that connects to the girth.
Blue is a snap, which connects to
Pink, which goes through
Green, a ring on each end of
Gold, a strap that goes over the poll.
Pink continues ends in a snap which connects to
Purple, the bit.
This might sound complicated, but it's really not. In the above picture, everything is pretty slack because he's stretching nicely.
Now, what if he picks his head up?
When his head goes up, pink and red get pulled taut. This puts pressure on the bit and poll.
When he stretches again...
There's no elastic, so there's a fixed length between the girth and the bit. The higher the horse's head, the more tension on the chambon, and the stronger the pressure to lower the head.
Now here's the thing. It also encourages forward, not just down. If the horse tries to curl back, it's going to take up more of that fixed length of pink and red, which will add pressure. When the horse stretched forward, pressure releases and the horse learns that he did the right thing.
Very simple and very effective. I got my chambon (which is admittedly not a very high quality one) for $8 in a clearance sale online. They generally run $30-$40. If your horse needs help with stretching down and out, it's worth it.