The best feed additive I've found for the coat is Uckele's CocoSoya. It is much higher quality than corn oil - the horse gets more benefit from it. You basically get a long more bang for your buck.
Farnham's Super 14 product is also very good. If a horse is slow to shed out in the spring feeding the Super 14 will get them to a show quality coat very quickly, IME.
I also feed fortified rice bran which helps a lot.
I also agree about not many baths (if I use shampoo I dilute it 1:10 with water), and lots of brushing.
Buckskins can be difficult to get a really high shine on and the CocoSoya keeps my guy with a very short, super high gloss coat. I couldn't recommend any single product more than it. You can find it at www.uckele.com