This is my favorite thing to do. I always love to do "tail projects."
One thing you should always do is Condition, condition, condition. I personally brush my horses tail every other day. I also keep it braided for about a week then take it out and brush and leave it down for a while. I also like to use MTG it definitely makes a difference in tail length. I use it on and off. When I don't use MTG I use the Infusium leave in hair conditioner. I also trim about two centimeters every three months off and I definitely makes a difference.
I've gotten my bestfriends horses tail about four inches past her hocks when it was just barely brushing her hocks. And my horse Comanche's tail is about a centimeter away from touching the ground.
"A good rider can hear his horse speak to him. A great rider can hear his horse whisper."