Yes what Samantha said - I keep a hand on their butt while I go behind and I also GENTLY take a hold of the tail hairs and walk close behind their butt with it - I release when I am on the other side. The closer you are to the butt of a horse - the less it would hurt if you got kicked. There is no space for the wind up. If it kicks - it is more of a push. So don't go WAY around - stay tight on the hind end. If you are working on the tail - always work from the side. If it lifts it's back leg (a typical resting position) on the side you are working on - gently pull the tail to that side and shift the weight to the opposite side. If it won't shift it's weight - then change sides and brush the tail from the other side. You should always ask the owner if the horse kicks. But I haven't seen a horse kick out in years...especially not a broke horse. But having said that - I wouldn't go out of my to run behind a strange horse and scare them as a kick out could be its reaction. Build your confidence on a safe horse that is known to not kick. You will soon be a pro. The closer you are - the closer you get!
For starters you could try where before you go behind them drag your hand all the way down them...like start by putting your hand on their neck and then slowly walk to their hind quarters and talk to them while you doing it. Like.."ok I'm gonna walk behind you for a minute" just to verbally say it so they can here you moving.