Tradeskills

With the goal of obtaining levels as quickly as possible it is recommended that if you decide to pursue tradeskills you choose those which will support the process.
Herbalism can be used to support Alchemy or add to your gold through sales on the Auction House or to vendors. Lifeblood will prevent you from dying so easily.
Mining can be used to augment your income by selling the materials on the Auction House or to vendors. Toughness increases your health.
Alchemy when combined with Herbalism provides many methods of negating damage done by mobs, especially casters like Shamans who can be more easily handled if you have Nature Resistance potions available.
Skinning is a good money maker as throughout your travels you will fight many beasts. It also won’t take much time, only 1-5 seconds after every beast. Miners and herbalists have to travel all the way to the nodes.
Tailoring might be a good choice if you are a caster or you just want access to cheap bags. Otherwise the cloth is better used for First Aid.
Blacksmithing is not recommended during the course of these guides as long term development is involved and costly.
Leatherworking is not recommended during the course of these guides as long term development is involved and costly.
Engineering is not recommended during the course of these guides as long term development is involved and costly.
EXCEPTION: If you have Mining and only use Engineering to craft ammunition (for a gun-using hunter) or explosives (dynamite and bombs), then this may be a useful tradeskill. However, it should only be used to assist the leveling process and should not be focused upon as a skill. In other words, only use it for supplying essentials for your leveling (mainly gun ammunition) and not for any other purpose.
Enchanting is not recommended during the course of these guides as long term development is involved and costly.
Jewelcrafting is not recommended during the course of these guides as long term development is involved and costly.
EXCEPTION: If you have Mining and only use Jewelcrafting to craft stone statues (for additional in-combat healing), then this may be a useful tradeskill. However, like Engineering above, it should not be a leveling focus and should only be used to assist with power leveling. Doing any more than this will become costly and should not be undertaken during the power-leveling process.
First Aid should be trained no matter what class you are playing. It won’t yield much money, but it is inexpensive to train and a great help to reduce downtime.
Cooking benefits all classes, especially melee and hunters (it is a free source of food). Until higher levels, the benefits it provides are almost exclusively stamina buffs. While more stamina is always nice, cooking is not essential to leveling. Considering the time investment required, if you pick up skinning then cooking would absolutely be a wise skill to level prior to hitting your level cap, as beasts often drop meat useful for skilling up.
Fishing requires much time and does not yield much profit until its later levels. Unless you enjoy it for its own sake, avoid seriously delving into fishing until your level limit. It is not a bad idea to do a little when starting out a new character on a new server, though – the gray items you fish up sell for good money to a starter, which can help you out with your other tradeskills.