There are only really two ways to "powerlevel" in WoW.
1) Higher level mage in party AoE's the crap out of everything around you. The killing rate outpaces the fact that you only get a small percent of the EXP because your partymate is much higher level.
2) Healer stays out of party and heals you as you fight. Most beneficial to mages so they can AoE with reckless abandon without worry of dying because the healer is higher level and probably has tons of mana for the level monsters you're killing. This works awesome for powerleveling, a high level healer can get a mage up a few levels FAST because the mage can provide plenty of offense and the healer isn't in the party, so doesn't soak up any of the experience. Works okay for rogues and warriors too, but for something like a warlock, it's not so much a big deal, more of a safety net, but doesn't let you level too much faster because you can't AoE (you'll be interrupted too much and a priest not in the party cannot cast PW:S on you)
Aside from that it's just learning the quests and completing them in an efficient manner. If moving to a server with guildmates you know you will be playing with, get them to make you some bags early. Silk is usually cheap, so 10 slotters should be pretty easy for someone to make for you. Just go quest and don't come back until your bags are full. When I level up a new character in Barrens, I will frequently have 5 or 6 quests to turn in at once. One time I even turned in 8 in one trip back. You move along so quickly at that time, you can even skip a set of skills and be okay, like don't get your level 14 skills until you turn 16, as this saves you a bunch of downtime while flying around. Stuff is so easy to kill at that point, your skills aren't crucial to your EXP rate.
Also, a particular class will have something that holds back their leveling rate. For casters it's usually mana. Mana is the limit of your kills per unit time, and for melee classes it's health. If you're traveling back to town and have full health/mana, kills eomthing on the way (hit it, then turn and walk backwards towards the town as you are fighting). For mana classes, be conservative on mana, with knowledge that it is your limitation of your killing rate. Finishing with full health and 25% mana means you have to wait for mana, but if you finish with 60% health and mana, you end up waiting less time overall (mages can mostly disregard that as they can eat and drink for free).