How can I counter Thornmail?
Thornmail Unique: On being hit, returns 30% of damage, before any reductions such as armor, as magic damage. At least 15 damage must be received.
As an AD Carry that relies in auto-attacks (eg.Ashe, Tryndamere, Master Yi, etc), What's the better way to counter Thornmail? (Providing some kind of source will be of great help, not only beliefs and guesses)
There are really only four things you can do as an AD carry against an enemy with Thornmail.
Get Armor Pen: Thornmail offers the highest armor rating in the game of I believe 100. This armor alone is going to be offsetting your damage pretty badly. Armor pen will only help you kill him faster, while he kills you... Thus bringing me to numbers 2 and 3.
Bump up your lifesteal: Items like bloodthirster boost your lifesteal, thus nullifying a bit of that damage you are getting back. The trick is to try to be taking away in lifesteal a good portion of what you are losing in thornmail damage.
Build up a good health pool: Something that a lot of AD carries don't tend to do is focus on their health, because frankly who cares. You're supposed to be dealing a ton of damage so you always boost your AD and ArP. However, when Thornmail is present try to grab some items that give you a health boost. As Ashe if someone has thornmail I usually grab a Banshees Veil.
Leave him be: You shouldn't be focusing someone who dropped a bunch of money on Thornmail anyway. (Usually it will be their tank and if its not thornmail won't save them from your damage output anyway). Try to just stay out of his way and focus down the other enemys. Let your casters deal with the tank, because Thornamil gransts 0 MR. I know with Rammus and Shen this can be hard becasue of their taunts, but leave them till last and let your team help you out.
Also important to note, Thornmail is returning Magic Damage, so any magic resitance you can build up will decrease the returned damage. So possibly a Hexdrinker might also help you out a bit. As does Banshees Veil.