I just passed 500k gold the other day. I attribute my success so far to doing several things:
First, I make a game of it. How high can I push the number? This removes the chore aspect of gold making, turning it into a means unto itself.
Second, I do not bemoan lost opportunities. I didn't get into gold making until about 4.2, so I missed out on high profit margins for JC, PvP gear, consumables, and Glyphmas 2010. I also missed out on the ore shuffle with high vendor prices. Nevermind all that. The inspiration struck late, but it struck deep. I now prioritize gold making over raiding and even PvP. (Though I really wish we could get a BG team back together.) Worrying about missed opportunity paralyzes. Focus on the next opportunities.
Third, I tap many markets. I'm not active in all of them right now, given the lull, but I dove in to just about every market. Glyphs have been my main income overall and I've almost doubled it by tapping the Horde side as well. More importantly, though, I was able to shift my focus from one profession or market to another, whichever presented the most profit. My most noticeable jumps in gold have been from enchants and enchanting mats, and from a few pricy flips (mostly Vials, some pets). PvP gear was steady mid-4.3, but prone to flooding. JC rare cuts are still good enough, just fluctuating weirdly. The JC epic market I'm again late into, but I'm still making gold from it. Leg armor and Buckles always sell for a high profit. Engineering and Dreamcloth I've given up on, and Transmute Alchemy mostly just supports JC and grinds out Volatile Air or Water (Truegold is too cheap). I even tested Cooking other things than Fortune Cookies (Didn't go well). All the markets fluctuate, and I'm ready to adjust. The one market I don't care for is transmog, though I respect the profit gains I hear about.
Fourth, I buy low. When the price is right, I take it all. Buying expensive mats cuts too far into profit, so I steadily buy up the cheapest mats throughout the week. Pick the low-hanging fruit. Old-world herbs, pigments, or even inks will sell for dirt cheap on occasion, as new scribes level. Whiptail drops drastically in price often enough. I twice bought hundreds of stacks of Whiptail off the Horde AH and many times off the Ally AH when some botters/farmers/dupers, whatever, flooded it. The first time I bought up the Horde AH, I spent about 8k, and made about 50k gold profit over the following 6 weeks. Savage Leather will accrue for cheap if you just buy up the cheapest few once or twice a day, and will often flip as Heavy Savage Leather for a 20-30% profit, as Pristine Hide for 30-40%, and as Leg Armor for 50%+. Embersilk Cloth works the same way. Skimming the cheapest mats off the AH throughout the week is especially vital for Volatiles and Chaos Orbs. Just be persistent in buying low and selling high will take care of itself.
I've now met my gold making goal for Mists preparation, so I'll begin reducing Cata specific stock that will be devalued and focus more on leveling alts. I have a 66 Rogue, a 35 Warrior, and a new Mage on the Alliance. My old Mage is purely a bank toon for now, and will be re-rolled as a Monk. I also have a 33 Druid on the Horde. As many characters as I can level and max professions on will be that many more opportunities to strike gold with in Mists.
Currently, my planned character/profession setup looks like this:
Kalven - LW/Eng
Kalmali - JC/Ench
Kalyka - BS/Tailor
Kalare - Insc/Alch(Transmute)
Kalyssa (66) - Herbalism/Eng
Kalida (35) - Mining/Skinning
Kaldwell (6) -
Kaledonia - Alch (Elixir)
Kallahan - Insc/Alch (Transmute)
Kallonika (33) - Herb/Alch (Potion)
The characters and professions in bold are maxed, the ones in italics are proposed or will be rolled in Mists.
I believe I will gold cap easily in Mists, so my next long term goal is actually 2 million. It might seem weird to skip past the technical cap as a goal, but it's just a number. Gold making has an exponential nature to it that can make certain jumps easier than others. For me, going from 2-3k to 10k gold was about what going from 20k to 100k was like, as was turning 100k into 500k. There were some stagnant times, but I think some of those were caused by increased spending, others by large investments (for good or bad), others by inactivity (like this weekend), still others by AH PvP. There are many different variables at work, and responding to them quickly and effectively has been, I think, the driving factor in my success so far.