World of Warcraft Guides
What's a clear guide one can follow that focuses on efficiently leveling up in WoW?
Jame's guides are rock solid!
I can verify that if you follow his guides, his XP estimates are not too far off. They are especially great when having to level an alt. Though this may change towards Cataclysm
The site also gives some general tips for leveling. My advice: follow them!