The overall goal of this strategy is to sell two T5 crafted items a week, each for around 1,500 gold. On the other hand, you can sell one T6 item a week for around 3,000 gold. Let's look at how you do this.
The first thing you want to get your hands on are as many T5 / T6 patterns as you can use. Learn them and be happy, because you'll be making a ton of these items. These patterns are BoP and are random drops in a T5 / T6 instance. I'm a blacksmith so I'll be using blacksmithing as an example. The principal T5 items I make are the Red Belt of Battle and the Belt of the Guardian. For T6 items I make the Dawnsteel Bracers and Swiftsteel Bracers.
The mats for the items need to be farmed by you or bought off the AH. If you are good at farming you'll not have too much of a problem getting them together. On my T6 warrior it takes about 2 1/2 hours to farm the mats for the Red Belt of Battle, my biggest seller. However your mileage may vary.
Of course besides the raw materials you're going to need the Nether Vortexes or Hearts of Darkness. The Nether Vortexes should be purchased with the badges you get from farming Karazhan, other raids, and heroics. It is pretty easy to get 30 badges a week (which allows you to make one T5 item). Kara will give you 22 badges, and the other 8 can come from clearing Gruul's Lair (5 badges) and Magtheridon (3 badges). Since the goal is to make two T5 items, you can either buy the additional two Vortexes off the AH or farm out the remaining 30 badges. If each day you run the daily heroic quest, you'll get at least 35 badges from doing just that.
Once you have the materials you need to sell the items. This can be the hardest part, or the easiest part. It all depends on what the market is like at the time, and what players are wanting. I've found that the market is best for selling these items on weeknights during "prime time," or from about 6 p.m. server until 10 p.m. server. If you're like me, you have scheduled raids during part of this time. That's where the second account comes in.
Make a bank character on your second account and get him to a Stormwind or Ironforge. Make a macro that announces your goods to the trade channel, and use the macro once every 10 minutes. You will get a lot of questions of "how much?" and "omg that's too much you n00b." Ignore the haters because you will eventually sell it, and you'll have stacks of gold compared to them.
When it is not prime time, I recommend that you put your crafted goods up on the AH for sale. There are always the odd players that won't log in until 2 a.m. in the morning, and you want to be able to get to them as well. Additionally if you have two items the best thing to do is to put one up on the AH, and keep the other around to sell via the trade channel.
With all that said you should be able to move two of your products each week for around 1,500g each, netting you a total of 3,000g. Do this each week for a month, and you'll make 12,000g. For me raiding costs about 2,000g a month, so in the end I'm left with 10,000g sitting on me. Not too bad.
Of course, there are some holes in all this. First, this strategy is dependent on there being a demand for your (theoretically) unlimited supply. You want to be sure that you are not flooding the market and eliminating demand for your products. And at the same time, you want to create an "illusion" of demand if possible. To keep demand high by normal economic forces I recommend that you do not sell more than two items a week. To keep it high artificially, I've had a lot of success by saying things in the trade channel like "One more Red Belt of Battle left for the week, get it now or you'll have to wait." People don't want to wait, especially for things they get in a game.
While creating artificial demand might be an underhanded practice in the real world and get you in some trouble depending on how you go about it, this is a game world, and there is nothing wrong with using it as a strategy. What you don't want to do is take people's gold without giving them something in return – Blizzard will ban you very fast.
Another possible source of income with these T5 patterns is making them when other people have the mats. I typically charge a combine fee of 300g for a T5 pattern, and 600g for a T6 pattern. If I'm aggressive in my advertising of these patterns, I can get about an additional 1,200g a week from just doing the combines.
Eventually this source of gold is going to dry up. Chances are it'll stop becoming viable shortly before Wrath of the Lich King. However at that point there are going to be other strategies for making a buck or two in game (namely gathering and selling mats necessary to level up the new inscription profession).
Take all this with a grain of salt if you want to, but it has worked for me. In the past two months I've netted over 50,000g, and currently have 28,000g in the bank and another 2,000g floating around in items that I'm selling on the AH (the rest of the money has gone towards buying my many alts epic flying mounts). With a bit of patience and work, you'll be able to see these kinds of numbers too.