Talking with Quadraphonic about leveling demon I decided to post quickly about what I think the most efficient way to level is.
Demon is a good tree but lacks power til around level 56. Demonic Frenzy (up to 50% more attack power for your pet) doesn't come available til that point and can really boost the DPS of your pet.
Lvl 56 Spec Link (http://www.worldofwarcraft.com/info/classes/warlock/talents.html?0500000000000000000002050030133250101501351000000000000000000000)
At this point you can still DoT instantly and keep going for a while (mana feed is key for this). There is 1 extra point that you can use to either build up your affliction, demon, or destruction tree. Our goal is 3 more points in demon (all in Demonic Resilience), 6 points in Affliction and 11 in Destruction. This is variable, but we do want at least 44 in demon for a demon-specced lock. The spec I used to level was:
My Spec (http://www.worldofwarcraft.com/info/classes/warlock/talents.html?0500100000000000000002050030133250101531351505000010000000000000)
This was also a very fun pvp spec :)
Good luck locks :D