Might be a repost, but I searched and couldn't really find anything pertaining to this specific type of power leveling.
What you need:
Warlock or shadow priest(lvl 70 pref.)
You, an alt, friend, what ever, this paticular guide refers to 20-40lvl using SM:CATH, so any lvl between 20-40 is fine, though this works with other levels and other instances, its harder.
Heres my video guide explaining it:
[ame]http://www.youtube.com/watch?v=Iw2NbOQleag[/ame]
mirror
Basically, you dot things then run out before they finish dotting, any person in the instance will get full xp, as if they killed it themselves. For a lvl 20, a SM:Cath trash mob is something around 600-1.2k xp roughly per. This may be common sense, it may be new to you, either way it got my alt from 20-40 in less then 9hours player.
Probably old news to any pre-bc WoW player, but to those of you who are new, its nice to know. Just trying to do my part in helping.