This method will get you from level 15 to level 70 in no time. However it will be tricky if you are not levelling a paladin, or you do not have some kind of aoe ability. I'm going to divide it into sections by levels.
First things first, for the 1-55 guide, you will need a level 70 class on another account that can dot in some way. Shadow priests and warlocks work best, although i'm sure other classes can do it just as well. If you don't have one yourself, you'll need a friend.
For the 55-70 guide, you need a warlock friend to summon you to outlands, and a prot paladin and mage friend to powerlevel you. If you are not playing a healing class, you may need a healer too. Athene powerlevelled paladins and druids.
so to start:
1-15: Just do your normal starting area quests, it doesn't take that long, you shouldn't need help with this.
15-30: Stockades (If you are a horde character, you're kinda stuffed, although you may be able to replicated it in SFK).
Invite your alt to group, and both you and your boosting warlock/SP enter the instance. The warlock then pulls the first room on left, first room on right, and any in the corridor. The boosted character must then run into one of the rooms the warlock has just pulled. The warlock then dots every mob he pulled, and runs out of the instance before they start dying. The alt then just needs to stay alive until all the mobs die, if one somehow survives you can just run out of the instance, but for every mob that dies, you'll get 500-700 xp unrested.
Basically you repeat this, with the warlock/sp pulling 5-8 mobs to the instance entrance, dotting, then running out. Make sure the alt you're boosting gets in combat, the mobs just need to run past him to get him into combat, so i often stick him in the corridor then run into a room. This works very well when levelling paladins, because if you spec prot, you can normally just bubble, consecrate, and finish off any mobs with a sliver of health left.
It's not a good idea to pull the right corridor, because many mobs stun, and will screw you over. Just pull the first corridor and the left corridor, then go outside, reset instance and repeat.
25-40: Scarlet Monastery Cathedral
Basically it's the same as above, except this time, you will want to KILL any casters before you get to the instance portal. The casters have awful habits of switching targets, which will kill your alt and cause long corpse runs. But again, you just pull 5-8 MELEE mobs to the instance portal, alt runs to a safe spot, then you dot them and run out. Again the alt will get a significant amount of xp each time. Do NOT pull from the cathedral, just reset instance and start afresh. Healing mobs will really screw up this method.
38-50:Zul'Farrak
Same method again, except this time, you'll want to nuke the witch doctors and shadow priests, because they will screw you over. At this point it becomes a lot harder, it's best if you're playing a warlock, so that you can pull the mobs with one cast of siphon life, and that should keep you alive until you reach the instance entrance. This is a lot tricker to pull off, but it's doable with practice. It may better for you if you just boost your character to 50 normally, with only the 2 of you the alt will get good xp.
50-55: Blackrock Depths, Stratholme, or Scholomance.
At this point it will get increasingly harder, but you only need to get to 55 (58 if you're wanting to level in outlands normally). Basically just find an instance where you can pull off the methods above, it varies depending on class and person, but you should have had plenty of practice with it by now. Again, you may find it easier just boosting your character through blackrock depths with a warrior or something, so it's up to you whether you want to continue using the hard method.
55-70: Slave Pens
This is where it gets interesting. You'll need a 70 prot paladin, a 70 aoer (warlock or mage), and a healer (which can be the alt you're boosting). Now, the mobs in slave pens are level 62-64. This means that a level 70 would still see them as green. So this is where the method changes. Your boosting alt needs an aoe ability, i'll assume you're levelling a paladin like athene was.
Take the alt, the mage, and the prot paladin into the instance. The prot paladin should pull 2-3 packs, basically as many as the mage can kill in one minute (this is important). It's important that the paladin and mage don't use any abilities to damage the mobs in any way while pulling them, body pulling is safest.
KICK the alt out of the group, the alt should use their aoe ability (consecration for example) as soon as they are kicked. This will tap all the mobs. The prot paladin must then instantly drop consecration to take aggro from all the mobs, or the alt will be in trouble. The mage then has 1 minute to kill all the mobs before the alt is kicked from the instance. As soon as they're dead, reinvite the alt.
Because the alt is within 8 levels of the mobs, and so are the people assisting, the xp penalty for assisting with the kills will be very small, so the alt will get an insane amount of xp for every pull like this (imagine that you pull 12 elite mobs - the alt would get xp for 6 elite mobs or so).
The better geared the paladin and mage are, the better the xp.
This levelling method isn't easy all in all, but it will level you extremely fast.