Please add any criticisms or obvious missing methods to the discussion page before adding directly to the answer article.
Starting out Edit
The easiest, if not fastest way to make money at first is to just find things you think you can kill (your level or lower), kill them, hope they drop something, and sell it. The second, slightly better way, is to do quests which will either reward you with money, items, or other quests. Quests also give you experience and usually reputation which will help you in other ways. If you get items as rewards from quests, you can use them or sell them. Yet another way to make money early on is with professions.
Remember, to make money you also have to save money. Don't buy any armor or weapons from a vendor that is limited supply unless you have a good buffer. They are nice, but usually over-priced. Also, you really shouldn't buy any items from the auction house unless you are a serious twinker or at least level 20. Some people say you shouldn't buy anything until you hit the level cap, but that isn't much fun.
Making money with professions Edit
The basic strategy for making money with professions is to gather and sell or to make and sell. If you are very social, you can make money on tips for performing profession related services (disenchanting, prospecting, smelting, etc.).
If you learned enchanting as one of your professions you can disenchant (a subskill of enchanting) uncommon quality or better items for ingredients that can be used or sold. If you have the right formulas to match ingredients you have accumulated, you can try to sell the enchants.
If you are social and people trust you, you can sometimes make money on tips for disenchanting things for people.
Be warned, if you aren't careful, enchanting can be very slow to improve and costly.
Gathering: Herbalism, Mining, and Skinning Edit
Making consumables: Alchemy and Inscription Edit
Alchemy is by far one of the best money-making professions. Inscription is just so-so, but has improved with dual-speccing. Research at the auction house and find what sells for the best margin (average sale price over ingredient cost) and sell that stuff.
Inscription is not as good for making money, but the market is volatile such that sometimes you can sell stuff for alot of money that has cheap materials, but you have to follow the AH prices carefully and be careful of competition.
Making durable goods: Blacksmithing, Jewelcrafting, Leatherworking, and Tailoring Edit
As always, research at the auction house and find what sells for the best margin (average sale price over ingredient cost) and sell that stuff. Be careful about listing fees for items as they can eat into your profit. Selling directly to players avoids listing fees, but can be more time consuming.
Blacksmithing is generally agreed to be the least profitable profession until later on.
Secondary: Cooking, First Aid, Fishing Edit
Cooking and First Aid work like other consumable making professions for money making. If you're lucky you will get rare recipes that can make alot of money from items like [Strong Anti-Venom] and [Savory Deviate Delight] .
Rogue: Lockpicking Edit
Making money on the Auction house Edit
Making money on the auction house is a guarded secret among many players, so you'll only get an overview here in order to not piss off too many players.
Basically the secret is simple: buy low and sell high. Obvious, but the secret is really in the details. So the big tip is this: get the Auctioneer AddOn and learn how to use it. Seriously, that should be enough to get you started. It is recommended to start with a safe buffer of about 5 gold. Don't buy anything to resell unless it has a market value of at least 3 times what you bought it for.
One last auction tip (this is really going to make people mad that this is out in public): if you're going to buy items to resell, stick to the items with low listing fees. That way you can try to resell them (put them on auction) many times without costing too much. Items with low listing fees tend to be glyphs, recipes, some profession materials, and odd quest items.