I can't remember where I saw it but I did see an ad for a DVD that teaches one how to trail ride
It was pretty expensive so it must be good
I am sorry but I've been trail riding more than 50 years. Just get the horse out with a seasoned & well grounded trail horse/rider.
If the horse has to be put in an arena before being loaded up and then embarassingly lunged at the trailer before mounting, it's the rider that's not ready to get out in the woods and meet the Marlboro Man or Bigfoot.
All the lunging in the world isn't going to prepare a horse for coming up on a rattlesnake or a bear --- both of which I have done (rattlesnakes more than once). Nobody panicked but we sure did back ourselves out of the snake situations.
Oh yeah, knowing how to back up is pretty important. Especially if you're on a rocky trail that's too narrow to turn around and there's a BIG Diamond Back peacefully sunning himself thirty feet in front of you, as you round the bend. Unless you come closer then things won't be so peaceful
Thankfully the bear was "only" a fresh steaming pile of dung so the bear was watching us from some hidden vantage point. Yes, the horses were spooked alright but stayed calm because we were well-seasoned and knew how to keep them calm -- it wasn't by lunging them
At the horse trailer--------------
Don't make a trail riding mountain out of a molehill; just go with a seasoned horse, forget about perfect show ring posture and try really hard to just smell the roses. The more confidence YOU have and are able to relax, the more the horse will enjoy life in the real world.