Went to a lecture once and a vet specialising in dermatology swore by Avon's Skin So Soft; so bought some. Result? Horse allergic to it. Advice: do a skin test first, as it obviously works for some but not all horses.
I had a Boett rug for my gelding. It was very good, but he is deep with mutton withers and the rug was too tight here. So I cut the seam and put extra fabric in to make it bigger. It worked.
Now he doesn't need a rug at all. A combination of massage where the horse gets sweet itch, and Pig Oil...