I'm definitely not half as experienced as the people here, and I agree that Weezilla's post was absolutely fantastic. I just wanted to add something that could help when you're riding him, although it might already have been mentioned.
I was stuck, during a clinic, on a horse who was in a fairly harsh bit and was very forward and incredibly high headed. I, being used to the horse who I trained, had only a slight idea how to control him.. since Major is the horse who, with a slight movement of your outside fingers, puts his head down in a nice frame. In order to get Whisper (the horse in question) to get his head down, I had to constantly half half, and as counterproductive as it sounded to me, I had to make him go more forward with my legs.
By the end of that week, I had him trotting and even cantering in a nice frame.. when previously, he was high headed and I was always pretty anxious for canter work since he enjoyed running off and bucking.
When I put what I learned to the test on Major, I was absolutely thrilled. There was no change really in his frame, but it only took a teensy bit of leg to get him engaging his hind end, which is something that usually requires a lot of effort.
Also, I once rode Major in side reins, but they were left loose and I only lightly played with them if I needed to.
Anyways, I agree with everything said here about the hindend engagement and driving him into the bit... it makes such a big difference in any horse. :)
Forever loved, never forgotten; my beautiful Indie. <3 Hoofprints on my heart.