Of course, if you can farm materials, you should probably sell them as soon as possible and use that money to buy 10 times the quantity you sold once the prices go down. Never forget that farmed materials aren't worth what you paid for them; they're worth what you could sell them for.
Priority One: Farm
Considering this crazy inflation we'll see in the first few weeks of Mists, priority #1 should be farming. I wouldn't even consider leveling a crafting profession until the undercutting starts forcing you to relist what you farmed -- and even then, consider waiting. The way it works is that the sooner you level a profession, the more it will cost.
Of course, there can be a benefit from having a crafting profession like blacksmithing completed before most others in your server have it. In theory, you can make some of the best pre-raid gear and some very important consumables. The problem is that you will have to wait until enough people get to the new level cap and start generating demand for level 90 crafted goods before you start making a serious profit.
Priority #2 for making gold in Mists will be leveling your chosen crafting profession. You want to make sure you're able to meet the demand as people start hitting level 90. Hopefully at the same time, the farmers will start to catch up to the demand for raw mats. This period is probably the most profitable for an early expansion crafter -- your competition is thinner than it will be, and the demand is steady. I made my first million gold selling gems, enchanting scrolls, and PvP gear in the second month of Cataclysm. Now, it takes me considerably longer to make a million gold because there are so many other crafters competing for a share of a shrinking market.
It's not free money, though. One thing you need to watch out for during the first few months is the inevitable drop in prices. They will start high, but as more people hit max level and decide to get in on some of the crazy farming income they've been hearing about for the last few weeks, prices on everything will tend to reduce. You will be more likely to see massive listings below the old market price. You can't let yourself get caught holding a lot of inventory when this happens. You should attempt to buy as slowly as possible while keeping your goods on the AH. Don't buy more than you'll be able to use in a week or so.
Even if you do find that the price for something you just stocked up on tanks, you might be able to liquidate it. This lowering of prices is something you can only see when you look at the average, but there will still be days when people are forced to pay top dollar for farmed goods, especially on the weekends when more casual profession-levelers tend to buy.
Don't forget new characters
There is always going to be demand for lower-level materials because there are always new characters being created, especially when there's a new race and class! If you're bored of farming, can't afford to level a crafting profession, and want to make money while you wait, you can try your hand at servicing this market. They will tend to want to buy enchants, Wrath of the Lich King gems eventually (which have no minimum ilevel), and materials to level their professions.
Farming low-level areas for low-level profession materials isn't a very good return in gold per hour, usually; however, what you can do is keep an eye out for them until the expansion launches and buy them whenever you see them for a reasonable price. If you need help figuring out what will be needed, a good place to start is the materials lists in the leveling guides.
Even if the demand for these doesn't spike, people will have more gold next expansion, and that means you'll likely be able sell almost anything you buy now for more.
Maximize your profits with advice from Gold Capped. Want to know the very best ways to earn 10,000 gold? Top gold making strategies for auctioneers? How about how to reach 1 million gold -- or how one player got there and then gave it all away? Fox and Basil are taking your questions at firstname.lastname@example.org and email@example.com.