This is pretty straight-forward and has worked since release but not many people seem to know about it even though it's still one of the best ways to gain weapon skills.
What you need:
-- A Warlock Friend of at least Level 28
-- Your Weapons
-- An Elemental or Demon NPC
You can probably already guess what I'm about to say... all you have to do is Banish (Banish - Spell - World of Warcraft) the NPC over and over while you swing away at the mob. You'll take no damage and you can keep doing it as long as the warlocks mana holds out. (Indefinitely)
In the following screenshot I'm simply banishing Fizzle Darkstorm's Imp (Fizzle Darkstorm - NPC - World of Warcraft)
Hopefully this doesn't require much further explanation. Enjoy!