For some reason I've always had a knack for braiding tails (manes, however are a totally different story haha) but I suck at french plaiting human hair :S
-Take small chuncks (about half a cm max) right from the underside of the tail when the soft skin is.
-I also leave a little room between the chunks on the underside so the hair going round stays more horizontal further down the tail and neater (this is probably why I can't french braid a humans hair very well
-I cross the hair from the existing plait before I add the chunk from underneth so the main plait looks tighter and you can play around with the tightness of the added strands to keep the braid straight.
-Also stand directly behind the horse so it doesnt go all wonky (talk to them and be aware when doing grumpy horses!!).
The top is the hardest bit to look neat, so for shows I would use a tinsy bit of hair spray or styling get at the top to get it lying flat and without the spunky stray bits pointing out everywhere! If im just plaiting for the heck of it I don't bother neatening the top.