Many of you probably noticed that if you shoot an enemy archer with a nice arrow, he or she will start using it against you. You can use this fact for farming arrows.

When you find yourself against an archer, shoot him with your nice arrow until he equips it. Try not to kill him. You can use a weak bow like a longbow. Or heal him if he is too weak. I think it works best if you shoot him after using calm or if you stagger him with Unrelenting Force or using a shield bash. If you are successful, you will notice that he has a different arrow equipped and start shooting new arrows at you.

This is good for duplicating arrows as well as leveling up your block/armor/restoration. It's good if you position yourself against a stone wall so that the arrows that miss you bounce off and fall on the ground.

Good for trolling fun sometimes.

I love getting a low-level Lydia doing this with daedric arrows. She'll get into a shooting match with some enemy archer, and there will be daedric arrows all over the place, as well as some added to hers. After you join the legion or the stormcloaks, the enemy camps will always have an essential leader that's great for this. Since he can't be killed, you can keep this up for (game) days if you want to, farming hundreds of arrows.