I have dealt with many barn sour horses. Mine is coming out of it actually.
Anyways, the way I do it, is
1. Lounge the horse (specially if they are high energy horse, I do not do this all the time though)
2. I start with a long lead rope, no chains or anything. I start to walk away from the barn, if there is tugging, pushing any type of undesirable behavior I simply put them to work, lounging, circles, halt back side side for about 8-10 minutes, then we walk as usually and start all over if they are doing something undesirable
3. Once I got the walk better I move to saddle, the minute I get a feet plant, constant calling with like hopping, prancing I put them to work for me, circles, figure eights, tight circle trotts then we go again after awhile horses will realize its easier to do what is asked, less work in the end.
Also this has to be done EVERYDAY, consistency is the KEY! Also don't put him right away when you get back work him for a little bit then put him back. So he associates the barn with work and going away the barn as fun time.
This is all Clint Anderson and John Lyons training, that I model after.