This method works best if you don’t have a lot of time to play and/or you want to level up more than 1 character as fast as possible. Using this method you should be able to increase leveling time by about 25-30%.
Gaining Rest: When your character is logged out, you become “rested”. The longer your character remains logged out, the more rested you become up to a certain maximum. Being logged out in a main city or at an Inn gains you rest at 4x the normal rate. Since it accumulates slowly, always try to log out in a big city or Inn.
Spending Rest: The maximum accumulated amount of rest will last for 1 1/2 levels. As you gain experience through killing mobs, you lose rest. While in a rested state you get 200% of normal mob killing experience.
The Method: Stay rested as much as possible – duh. If you don’t play much this is easy. If you play more, create and level more than 1 character at a time. Depending on how much you want to use rest experience and how much you play you might need 3 or more characters.
Generally I will start one character and level it to about 10. Park it at an Inn and make a new character. Play it to 10, park it and start the next. Then once I have 3 level 10 characters, I go back to the first one. That first character should have gained some rest while you were working on the other 2. When the first character uses up all of its rest, park it and log in with the next one.
You can round robin through your characters and generally get at least some rest accelerated leveling every time you play. Other advantages to this method are the way in which your characters can help one another. If one character finds a decent item that they cannot use, another of the character probably will. Just mail it to the correct character. You can also obtain a lot more of the professions (or all of them) so you can be a one person guild supplier for yourself.
Here is the official World of Warcraft site’s reference on rest.