I love most of the ideas here, just a few others for you to try:
Take your horse on some walks. I am not world's greatest rider, but I made a free gelding (terrified of the outside world) into an amazing trail horse by walking him. Sometimes we don't have seasoned horses to pony with, and hand walking will help. I would alternate ponying him with another horse and hand walking him until he is completely calm in hand. Once your horse can be walked out on the trail with you, try lunging and ground driving in unfamiliar places. Try getting on for short amounts of time and getting off, and then continue leading until you want to get on again.
Things that are scary should be introduced. I purposely introduced my horse to llamas, dogs, plastic bags that we passed on the trail. If he spooked at something, I would walk him back and forth past it again and again. Then, I would get on and pass it again and again until he was relaxed.
I would ride with others and make him listen to you. Let them lead and fall behind for awhile. Circle him and ask him to work like you would in the arena while the other horse is off in the distance. Make your horse take the lead and make him listen to you if the other horse starts going away, and make him leave the other horse, and then go back to him.
Start with the same trail, the same routine, every single time. Gradually build on that routine and introduce new things only after he does the regular routine well.
The patterns idea is really important, it helps a lot with building the relationship between you and the horse and teaching him he has to do what you tell him.
Major sacking out is also a good idea. Plastic bags, tarps, motorcycles, bicycles, loud music, water, etc.
Basically, if you can think of it, do it. The more different things you can do, the more it will bring out is natural inquisitiveness, build his respect for you, and create a work ethic that will translate to good trail horse. I think these are all good methods for novice horse riders, they make take longer but they produce really good results!