I noticed that a lot of my characters have extremely low defense skill for their level. I don't let myself get hit often so my defenses tend to be on the weak side. But I found a way to level this up very quickly and effectively while I'm not even at the keys.
First head to the Ironforge/Stormwind tram. Sit in the middle of the platform and find a move that will aggro an enemy but not do any damage. Moves like hibernate, polymorph, curse of exhaustion, work great. When you use them on a rat, it won't die, but when the effect wears off, it will run towards you and attack you nonstop. It will nip you over and over for about 1 hp damage each time. If you wait long enough, these little hits will boost your def skill.
Keep aggroing all the rats you can until you have a whole swarm of them nipping at your ankles. Now as long as you can survive a while, you can go do something else. The rats will keep boosting your def slowly. Just check back from time to time. 10 rats doing about 1 dps each may eventually kill some characters. When you come back you will have probably gained some def skill. You may also be dead, but oh well. If you want to do this without any armor on there's no risk at all of even losing durability damage.