One fall my mare scratched a significant portion of her tail out, so over the winter this is what I did:
1) I washed it with good shampoo, then let it set in conditioner for a while. Then I rinsed real good and let it dry.
2) Then I applied Shapley's MTG to it (the directions are pretty straightforward;)
3) Once her tail was good and dry (IMPORTANT), I braided it (braid it loosely near the dock, it doesn't need to be super tight, that will only break/stress the hairs). After that I folded her tail up on itself until it was a little "bun" like thing, then I slid a soccer sock on it and sewed some yarn through several times to hold it in real well. (One of my horses had a hard time keeping it on, so I used electrical tape wrapped around on top of the yarn, and this helped!)
Every 2 weeks or so, I'd take it out and redo it though, just to get the best results!
Just my 2 cents worth, but it did work really well for me! Her tail grew several inches and all those annoying short hairs on her dock grew out and made her tail MUCH thicker! Hope this helps and good luck!!:)
oh and one more thing, i usually only do this when the flies are gone, but if you need to do this before then, attach some strands from a mop to the tail, so she can swat those flies too!:)