The World of Warcraft economy is a fickle beast much like that of our real world economy. It operates on its own own rules and has over time become something that people study almost as carefully. For those people looking to make a lot of gold in the game, there are many ways to do so, not the least of which is to actually buy and sell in popular goods. However, the most popular and most utilized methods still remain as those that involve hunting and gathering goods. And extremely, it is the farmer that makes the gold in the game.
Farming itself is best for those characters who have reached levels at which they can kill mobs on their own and take away multiple items with each kill. Usually this works best for level 40 and up characters. For those that do decide to farm, the best way to go about this is to run incidents and kill everything you find multiple times, picking up loot repeatedly and looking for high ranking and epic items.
For a long time, this was the most popular method of winning gold for almost all level 60 players. Although there are now better methods to making gold for those players, including PvP and Honor System rewards. For those that decide to farm instead of gathering to make gold, there is as good chance that you may make more gold, but there is a guarantee that you'll spend more time on it. It really depends on what drops when you farm. You could make 20 gold in a day or 2000 if you're lucky.
You should farm for as long as it takes to fill up your bags and storage space and then send off your items to the Auction House. Sell everything you gather to the auction house, regardless of its rarity because it will ever sell there and will likely make more money.
The only downside to doing this is that you will pay a little bit of gold to hold these items and put them up for bid. Your best bet to offset this is to sell your items over the weekend. You should store all of your items from throughout the week and post them all for sale on the weekend when more than three times as many users are online. This will maximize the amount of people who see your items. Additionally, take the extra step to advertise your items and make sure everyone knows they are available.
In the end, the best way to approach farming is to spend large amounts of time raising a certain instance and picking up as many items as possible. However, it's a good idea to target lower level items at times as they take less time to pick up and add up quicker (without needing to recuperate too often. you do it properly.