Definitly brush the tail. Most horses don't mind it and actually enjoy it. Get a good stiff hair brush or mane and tail brush. Start at the top bushing the short hairs toward the center. Then go to the bottom and work your way back up. I grab the whole thing and brush it against my leg, turning it to get all the way through it (vida has a very thick tail). I stand to the side with my arm against her so she won't forget I'm there.
Just a heads up, during fly season its a good idea to put some on before you start, that way they wont be tempted to start stomping their feet while you are standing next to them and possibly stomping your foot by accident.
I also use a leave in conditioner type spray if its really tangled. Human type stuff is great and usually cheaper than the horse stuff. I like the Dove leave in conditioner. I have just started using some stuff called Africas Best Herbal oil. I found it in the African American hair products section at Wal Mart. Its suppose to stop breakage, I'll have to make up a review on it if it works good so keep an eye out for it.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France