Immediately after entering the Blackrock Depths there is an entrance way to your left. In order to gain access you will need to either pick the lock or have the Shadowforge key. (Hint: This key is obtained by completing a quest inside the Blackrock Depths). After gaining access to this area you can either go right or continue to head straight. For this technique you need to continue going straight to the area known as the Dark Iron Highway. This is the area that will allow us to make tons of gold mining ore. In this area there are numerous dark iron spawn points and on a single run you should be able to make one or two of them if your fast enough. You simply just mine the ore, leave, reset the instance, and repeat the process all over again. You will probably need 3 or 4 people for this to work. But, if you are a druid or rogue you can be stealthy and sneak in. Another trick is for hunters to dart in and feign death to escape the groups around the ore.
There may be times when you'll have to clear the mobs surrounding the ore. Other times the ore will be in back of so many mobs that you won't be able to reach it and you will be forced to reset the instance immediately. Sometimes the area will be completely clear for you, it's all completely random as to how many mobs are spawned in this area. The mobs in this area are all bloodhounds. They can be tricky to get by since they can see through stealth from a very long distance. They also have a powerful attack, but luckily they are weak in hit points. Going up against the bloodhounds is not recommended, but if you're careful enough you should be able to fend them off by yourself.
On average I can collect 30-40 dark iron ore per hour when I use this technique. Since the dark iron ore retails in the range of 1 - 5 gold a piece, it makes for an easy profit. I saved enough gold with this technique to get my epic mount in just a couple of days!