Ok, *tongue in cheek, here*...if your horse has "cankles" you must sweat them. The ankle area is typically bony, so any excess flesh is going to be mostly water and fat. Put mini-sweats on these (perhaps you could adapt human wristbands) and then work your horse 'til they have svelte little ankles, not cankles!
So I have NEVER put anything under my horse's tails, and I find that the best way to get them to lift their tail out and up is curiosity and natural tendency/nice hip. The horses with the poorer hips are not going to be able to flag their tails, and the effect won't be as nice.
The horse in my avatar, while not flagged (it is more like just hanging where it does), is just fine for a horse w/his short/weakish hip. If he had a better hip, he might just be able to hold his tail higher more naturally because he has the attitude to do so, and does so in a manner that he can, considering his build.
His little brother has a nicer hip, and holds his away from his body more so when he is interested in something. For example, his ears go up, his eyes light up, and his tail naturally raises a little, creating an overall pleasing, curving line through the body, something that is visible from a distance in a balanced horse. It IS preferable in that a balanced horse is more likely to be chosen in a halter class, but not just because the tail is "up."
Still not flagged, but you can see how he holds his tail a bit away from his body, which creates enough of a nice line, to me, compared to a clamped tail (mostly horses that are nervous will do this, or ones that just have the build to hold their tail flat, like his brother above).
This mare naturally holds her tail a little more high:
I have NEVER gingered a horse, EVER (not that mine have "that look" mine are not nearly that flagged), all this I have posted here is natural, and so is this:
(same horse)
I hope some of that made sense and you can forgive my silliness over the cankles. I don't think ankle sweats would be a good thing at all, seriously.
IF your horse had cankles.
Liz