Here are some unedited pictures of my mare (yes, I edit my pictures to make her look blacker, haha)...
When I first got her: http://i42.tinypic.com/302753s.jpg
After a short time on BaK + blackening shampoo: http://i42.tinypic.com/339u4xt.jpg
After a longer time on BaK + blackening shampoo: http://i44.tinypic.com/zob28x.jpg
After BaK, blackening shampoo, and a short time on night turnout: http://i42.tinypic.com/2j0foxv.jpg
Now, after BaK, blackening shampoo, and night turnout: http://i43.tinypic.com/2re312p.jpg
That's really all there is to it. Diet is the most important thing. Make sure he's on a high-quality feed and is getting all the nutrients he needs. Black-as-Knight helps tremendously. Aside from helping color, it also improves the quality of the coat, skin, and hooves.
Night turnout is the second most important thing... and if you can't do that, get him a fly sheet that blocks UV rays and some spray-on sunscreen (I use Coat Saver sunblock).
The third most important thing is to make sure sweat doesn't dry on him. Hose him off meticulously after every ride, and if he gets sweaty in his stall or pasture take him out and hose him off. This is a good thing to do even if you aren't worried about fading because the salt makes them itchy and uncomfortable.
The fourth most important thing is blackening shampoo. I use Emerald Black and I love it. I will say, blackening shampoos don't seem to make brown horses black, but they do seem to improve the color on horses that are already black by making them jet black or blue-black.
The mane and tail are the tricky part. They're still orange on my mare. They don't fall out and grow back like the coat does, so once they're faded they will be for a LONG time. I use regular human hair dye on my mare's mane and tail, and it works great. Just don't overdo it or it can thin out the hair.