A surprisingly efficient way to hone your tanking skills is PvP.
While you're unlikley to learn how to position mobs you will learn how to use the abilities that tend to just occupy a slot on your interface but never get used.
In PvP people are actually out there to get you. That's what PvP is all about. But it also happens more slowly. No boss slamming you for 100k damage or funny crap on the floor instagibbing you. You'll be stunned, slowed, rooted in place and watch that friendly healer go down costing you an avoidable death.
So you'll learn when to bubble, when to LoH, when to use Hand of Freedom and when to bubble your healer. You'll see that healer leap you out of a lost position, when you should run into the fray and what types of opponents are best gripped to your position by that DK.
Most important, you'll always play in an environment where you don't know beforehand what's going to happen, and that hones your most important tanking ability -- that of environmental awareness.