Kill Everything that Moves
When you are sent to an area to complete a quest, whether it's stealing pumpkins or killing, go above and beyond the call of duty. Kill everything that moves. This may make your questing in certain areas take longer, but it will be worth it in the long run. You will gain more experience and items, both integral elements to power leveling.
In areas where enemies repop quickly, stick around until their loot drops dry up. Once their loot drops to grey, or they stop dropping loot altogether, move on to the next farmable field or quest area. Killing time is best spent gaining both experience and loot, so move on when the loot drops decline.
Concentrate Your Questing to Individual Zones
When your quest log is full of tasks that take you all over World of Warcraft, remember that travel is the biggest time killer in the game. When you are less than level 40 and unmounted, you will find this time better spent concentrating on one areas' quests and banging them all out before moving on to another area. This way, you have maximum completion and are able to turn in multiple quests when you make the long trip back to town, thereby leveling in an efficient and timely manner. Use a guide or a source like thottbot.com to map out your best plan of attack according to where the enemies/quest items are, and how close they are to one other.
Stay in the Yellow
The only enemies worth killing are yellow and above, and green enemies, while easy to kill by the dozens, often provide few experience points, and loot that is often less than your needs. Certain classes, and certain players, can pull off the occasional orange or red kill, but they take a long time and the payoffs are not as much as you would expect in terms of experience. These kills may have better item drops.
Likewise, concentrate on yellow and above quests, and abandon greyed out quests every time you level. It may seem like the items or payoffs from these quests make them worth hanging on to, but when you are trying to get your character to level 60+ in a hurry, wasting time to complete these quests will detract from your time spent cleaning up on experience points and loot. Concentrating on yellow quests will mean you are receiving enough experience points for your efforts, and that the region in which you are questing is at an appropriate difficulty level.
Farmlands = Time to Farm
Areas that look like farmlands, typically populated by Gnoll, Undead, and Human enemies, are often the focus/center of quests involving theft, revenge, or destruction. Whenever you see a farm, stick around for a while and kill the enemies which populate the farmland as they repop.
As long as the enemies you are killing are yellow level, they will be a good source of quick experience and loot. As they repop, knock them down until your bags are full and you are becoming bored with killing, then finish the quest up and move on. Typically, these areas will also be a good place to come back and farm, especially later in the game when farming becomes critical to finishing up your character's gearing and advancement. Remember the farms whenever you are short on cash and need loot fast. And farming while questing will fill your bags with loot, and give your experience a boost.
The Auction is the Key to Earning Lots of Gold
You have to buy, and sell, loot in the auction house if you expect to keep your character well-geared and kicking butt. Selling items, especially those which you have made with your chosen professions, is a great way to make sure your character has the gold they need to keep their skills leveled up, to ensure that they have the proper gear, and to allow for some occasional splurge spending on fun stuff like pets, weapons and profession leveling.
Buying level appropriate gear will ensure character security, while the ability to buy ingredients necessary to raise your profession skill ratings will ensure your character stays in top form. Utilization of the auction house is a key component to power leveling your character. Make trips to major cities a part of your routine, and always check with your local auction house for anything you might want or need, to ensure that you find it at the best possible prices.
Don't Neglect Your Professions
Let professions reinforce your leveling, rather than becoming the reason for leveling. Often, players waste time collecting goods to raise their professions to learn how to make more stuff. If the items you are making aren't usable, because your level is too low, or you are not the right class or lack the skill to use the item, then you should not be in such a hurry to make these items. Everyone wants to have a high profession skill level as quickly as possible, but these skills are supposed to progress with your character, and should provide sellable or equipable items which will benefit your character. If your profession has gone past your character level, give it a rest for a while.
However, when you are ready to boost your professions nothing beats having lots of money from questing and farming, so you can buy the necessary items from the auction house and boost your skill levels 5 to 10 points at a time. Look for the skill which uses the least amount of items in its preparation, or the cheapest items to buy from the auction. Then buy these items and power level your professions!
Miners and leather workers can benefit from this method because they can raise their skill regardless of whether they can gather the items from the zones in which they originated. Even a new player can start smelting iron or working with medium leather and buy stacks of supplies from the auction house.
Streamline Your Talents Towards Maximum DPS
When in a dungeon, DPS (damage per second) is relegated to the tanks and warrior classes, those who hold the enemies in place with threat (agro) while everyone else blasts the heck out of these elite beasts.
However, when characters are running around in the field, completing quests and killing beasts of all shapes, sizes, and levels, it is important to max out your DPS to ensure that you speed through enemies, and thereby speed up the leveling process. A high DPS means enemies fall quicker, and you are able to handle multiple enemies by putting them down with maximum damage. Streamline your talents towards maximum DPS rather than protection or other boosts, because this will ensure you have the muscle to put down whatever comes your way.
Maxing out DPS may make life a bit harder for Warlocks and Hunters, as high DPS typically means taking the monsters attention away from your pet and onto you, but you will learn to use this threat to your advantage as you level up, and enemies will often be caught in a deadly game of pickle between you and your pet. Max out your DPS with your weapons as well, because this will ensure that your character is doing all they can to kill enemies swiftly.
Keep Your Skills and Gear Up to Date
This is part auction house, part trainers, but you should definitely work on one side at a time, because this can be the costliest part of power leveling your character. The auction house provides a bounty of weapons and armor, mostly green but some blues you will be able to afford, which will keep your character current. This is most important in terms of weapons, because you want to keep your DPS as high as possible, so you must upgrade your weapon whenever possible. Only upgrade to an item which improves the buffs (attribute/skill bonuses) appropriate for your class. Just because something has more armor, or looks cooler, doesn't mean it will help your character power through the game. Look to raise your attribute buffs from gear whenever possible, and this will help you stay on top of your game.
As for skills, these upgrades are often costly, so you should only work on one side at a time. Quest, come back to town, hit the auction for new gear, quest, come back to town, train. Try to do both every time you come to town and you will soon find that you are lacking crucial gear and skills! When you go out and quest, find out approximately how much money you will need to up your skills, and don't come back to town until you've got the cash!
It is easier to raise skills every three levels or so, with the exception of levels like 20, 30, and 40 which often represent milestones in character advancement, and offer up new skills you just have to get as soon as possible. Dropping gold on a bunch of skill points to round out your character is far better than always being broke and skill-less.
Leave Discovery for Mid-Levels
There is a lot of running and filling in map locations which takes place during your time in the World of Warcraft, but this exploration should be left to the middle levels (30s.40s), when you are closer to being mounted, and can explore in a quicker manner. Concentrating quests into specific areas means less travel time and more concentrated experience points, so when someone offers you a quest which entails running across two zones to complete, say no thanks and move on.
If, however, the quest entails moving to another nearby zone, make sure that the enemies in the new zone are of an appropriate level (yellow), and that the quest is yielding yellow level experience. Only then should you go forth and move about, or else you may get stuck in a worthless zone slogging through character levels on what will feel like a never-ending quest.
Move About the World, Keep Things Fresh
When you are questing, you will run out of quests which are level appropriate, and this is a sure sign that it is time to move on. When this happens, look for a zone to move to which is near the area you are currently in, and which is populated with enemies in the yellow zone.
This doesn't mean traveling all over, looking for a place to post up, but rather looking for another good starting position (like The Barrens for Horde players), a place where level advancement simply means moving north, south, east or west to another zone, a place you can still return to without feeling like you are surrounded by lowbies and should have moved on long ago. Click through the maps in game, and see which areas serve as hubs to other zones, with 4 or more off-shooting roads leading to different surrounding areas. Finding these hub locations ensures that you don't spend an hour traveling around looking for enemies your own level, or quests for you to pick up, and once you have formed a network of flight paths surrounding this hub, questing in this region will be a breeze, and you will fly through character levels like never before.
If you have a guidebook, you know how easy it is to look this sort of stuff up and map out your questing. The guide has zones broken down by enemies and their levels, and easy to follow dots which show all the key areas to fill in on your zone map, taking the guesswork out of questing and finding new areas to use as power leveling grounds.
Utilize online resources like thottbot.com or the World Of Warcraft home pages, which offer forums and tips, and you will find a wealth of information in the form of maps, zone enemy breakdowns, gear lists, dungeon walk-throughs and more. These are an invaluable part of the game, and will help you cut down on time spent in the game running around blindly.
Using the guide to map out zones, and the online resources to show you about classes, dungeons, and gear, will ensure that your character is power leveled and looking good, and will allow you to make the most of your time spent playing World of Warcraft.