Long and low is achieved by asking the horse to stretch or reach into the bridle. I find it easiest to do on a large circle at the trot since it places the horse naturally into the proper bend and the inside leg to outside rein combination.
Pick up an nice forward trot on a long rein. By long, I mean long enough that he still can extend his head and neck, but you can still feel his mouth on the other end of the reins. Next, stay on a large circle and really concentrate on keeping your hands quiet. Do not at any time pull back on the reins. You want your horse thinking forward and stretching, not the other way around. Keep your hands out in front of the saddle, relaxed elbows and shoulders. Follow his motion with both hands evenly, none of that see sawing nonsense. For some horses, this is all it takes to encourage them to reach for the bit. If not, continue reading.
So now you have a nice circle going, but he's still star gazing. You can try two things. The first is spiral in/ spiral out. To spiral in, use the outside rein and leg. It sounds backwards, but it is effective. You want to think about shutting that outside door to keep him on the smaller circle. When you're ready to spiral back out, just reopen that door slowly. If he starts to chew softly on the bit, he's starting to think about using his back and stretching.
If you get that far, next thing to unlock that last piece is to transition within the trot. On that same circle, without changing tempo, ask for smaller steps, then larger, etc. Each time he is asked to shorten his frame and then be allowed to stretch out again, he will be more interested doing just that.
Once he reaches into that bit and you feel his back come up under you, you will proceed to walk around on air for the rest of the day since it is such a fantastic feeling. Ironically the horses seem to love it too which makes me wonder why they all seem to pitch such a fit about using themselves properly in the first place. Good luck!
You just have to see your distance...you don't have to like it.