So you are leveling your fifteenth alt and it's getting a bit dull? Well, you could do some stuff that makes leveling a bit faster. For instance, once you reach a high enough level to queue for a new dungeon, take a few minutes to travel around Azeroth and take all the quests you can for that dungeon. Take it one step further, even. Start leveling in zones connected to the right dungeons. You will get some more advanced quests and you will lose less time on traveling around.
Secondly, make use of the cloth turn in quests, if you have the cash. They are quick and easy to solve and will give you a nice experience boost. Just remember, only the first runecloth quest for each faction gives you experience. No use in doing it more than once.
Thirdly, once you get to 57, go to the Eastern Plageulands and do all the turn ins you can there. Dark Iron scraps, Cores of Elements, anything you can buy on the auction house, you buy and it will probably give you enough experience for the last level before outland. It usually does for me, but I'm not sure of the exact total you get.
Other than that, make not of quick and easy quests you can do just by traveling around or getting something of the auction house. They might not be worth it on this character, but you can get stuff in advance for the next character you will be leveling.
Now, if you really want to take it to extremes, try leveling a few alts at once. Leave them at inns so they can get rested bonus and simply queue a random dungeon ever now and then to make use of the bonus. You could be leveling one alt normally, and just do randoms every few days on others. It might not be the fastest way to get them all to the cap, but it will give you most for the least amount of time invested.
I am aware some of these things will change now that Cataclysm is around the corner, but if you have the right mindset, you can find similar opportunities even in the revamped world.