The principles of farming are very simple; go out, kill or gather materials that other people wish to buy. Essentially, you are providing a service to the many thousands of people who are not keen on having the patience to farm and gather the things they need and instead, they buy them. The only thing remaining is to know what the materials are that people are willing to pay for.
The gold guides usually always enjoy the talking about rare blues, pets, and other items that randomly, but share the common factor of having an extremely low drop rate. Take the Tiny Crimson Whelpling for example. It drops from mobs in the Wetlands, with a drop rate of one in a thousand kills. These are what I call sexy targets because they make people oogle and oooh but in truth you can spend hours upon hours killing the mobs and not see this item drop.
To me, this is marketing trick, letting people think there is an easy 1,000 gold to be had if you do it this way. Instead, you will probably only waste a day trying to get the item. I much prefer to farm for materials, rather than specific objects. Materials have the benefit of being universal, always needed for both old and new products.
Mining is a great profession to have because of this, as people will always need even the simplest of ores, copper, bronze, iron to level their blacksmithing and jewelcrafting. It is no coincidence that the one, most produced item in the game is the saronite bar. Of course, other gathering skills also deserve a mentioning, herbalism and skinning, both which can be used to gather valuable goods.
Combined with a useful crafting profession, say herbalism and inscription, you can also gather the materials, turn them into specific goods and then sell them. One of the most powerful selling items, at the moment, are the darkmoon cards, some which can sell from one thousand to five or even six thousand gold.
The possibilities are endless and in my personal view, a lot more meaningful then spending a day farming for an item that you might or might not get. It all simply depends on how aware you are and is a matter of trial and error. Observe the auction house, try to find items that seem to sell for decent prices, then go out and check wowhead, wowwiki, and thottbot where you can best find them and go out hunting.
As always, it all depends on yourself. Nothing ever comes for free and as you start out, it will require research and effort to get you going. But the rewards are good and guarantees that you need to a) not buy gold from sellers and b) never feel wanting. Trust me, this is a way with which people spend tens of thousands on flying choppers and traveling mammoths.