Primary professions often go together in pairs. While there are many debates running on what works best, here is what I've found compliments the skill sets together in the most lucrative way:

Alchemy / Herbalism, Blacksmithing / Mining, Engineering / Mining,

Jewelcrafting / Mining, Leatherworking / Skinning, Tailoring / Enchanting

Alchemy / Herbalism

If the point of primary professions, (besides adding to the enjoyment of the game,) is to make money, why take a profession like Alchemy without Herbalism? Sure, you could pay for herbs in order to make your potions, but isn't that kind of like putting the milk before the cow? Gathering herbs is easy at the lower levels & bundles will make you some cash on AH. Potions will also gather a price on AH especially from characters like warriors who have little in the way of healing spells or buffs. Alchemists can also transmute certain raw goods into other types of materials. Transmutation requires a Philosopher's Stone, which is created by the alchemist and carries a “cool down” rate of up to a few days, meaning once an alchemist has transmuted an item another can't be done until the cool down rate expires. Transmuted items, such as an arcanite crystal into an arcanite bar fetch a hefty price at AH.

Blacksmithing / Mining

Blacksmithing is one of those primary professions that has an exponential rate of return, but a slow growth rate in regards to money. Luckily your second profession Mining has a high potential for cash. There are some kinds of armor and weapons that cannot be found anywhere in the game other than through creation through a blacksmith, which carries a huge potential for AH profits. Also, blacksmiths can provide “extras” to existing armor and weapons, such as sharpening and weight stones, steel weapon chains, shield spikes and mithril spurs, to name a few. These are huge money making opportunities, everyone wants a pair of mithril spurs for their boots to increase their mount speed, or a weapon chain to keep themselves from being disarmed.

Engineering / Mining

Engineering is probably one of the most unique and interesting professions in WOW. It's also possibly one of the hardest, due to the fact that unlike most other professions, it requires a lot more raw materials and several more steps to create an item than any other profession. It also requires more bag space, so make friends with a tailor who can make you those, as well as mining bags and engineering bags. Like blacksmithing, the early levels of engineering can be hard to make money, so it's always nice to have mining to fall back on as your second primary profession.

Jewelcrafting / Mining

Jewelcrafting is possibly one of the most lucrative professions for low-level players in WOW. The jewelry you can make will almost always sell for something on AH. Again, mining is a good complimentary second profession and because of the way jewelcrafting works, I believe mining is a necessity. Instead of selling bundles of ore on the auction house, you can prospect the ore: (5 units of ore per one prospecting attempt,) copper, iron, tin, mithril, thorium, etc., and receive gems & jewels for your efforts. Put it all together, smelted bars and jewels, you'll make a piece of jewelry that will sell for anywhere from 10 silver to 50+ gold. Higher level craftspersons are able to cut gems for sockets, which makes insane cash on AH. All in all, it's a highly lucrative profession. Like engineering, you might want to get yourself a special set of bags, (a gem bag, mining bag,) to maximize space.

Leatherworking / Skinning

At low levels leatherworking and skinning are possibly the easiest profession sets available. Armed with your trusty skinning knife most of the furry or scaly beasts you come across will garner some hide or leather. You can combine pieces of lesser leather to create higher level leather goods, so my suggestion once you reach higher levels is to take the time to create higher leather goods out of you lesser ones. You'll make more money on AH doing it that way and you'll gain more leatherworking skill. Other ways to make money on AH are: making armor kits, riding crops and drum sets.

Tailoring / Enchanting

This combination of professions boosts your charisma modifier by two. Just kidding! I kid, I kid, there are three classes that can only use cloth armor: Mage, Priest, and Warlocks. Not only does this add up to big money in the bank, it adds up to a character that is loved and adored by all they come across. Play your character right & you'll get all manner of special considerations. Everybody needs enchantments, everybody needs bags. I combined these two professions together because tailors need certain enchanting supplies, it seems like the best combination available. Some people think that skinning is a better choice, but personally, you can buy leather a lot cheaper at AH than enchanting supplies. You can make specialty clothing, as well, which you can turn around and sell on AH or give away as gifts. On the enchanted side of things, I can't get over how much fun enchanting is. I suppose in the early stages you do miss out on some cash because your disenchanting some of the good drops you get, but really, that stuff isn't worth much anyways. A bonus for enchanters who have tailoring, early on in levels you can disenchant robes right away. (Jewelcrafting is another second primary profession that has this bonus for enchanters. Honestly, skinning? Sorry, I'll get off it now.) I've personally found that anything else I need as an enchanter I can usually get from the person I want to do the enchanting for, so the costs are relatively low, especially at lower-levels. ---

So I guess the secret is out. I have a character for each of these profession groupings. I realize people aren't always able to do this, WOW will only allow a person 10 characters / account, but if you're able to do this, that would be one of my last suggestions for primary professions. You'll always be gathering materials for your professions no matter which character you're playing. Whenever a character grabs materials for another character's profession, you just send it through the mail system to the appropriate toon. Simple, right?

The first 20 levels or so most of these professions offer a way to make a substantial chunk of change in order to get that first mount and really start going places. A lot of WOW is just a manner of personal opinion, but I think in this second article on WOW economics I've laid a sturdy foundation for the ordinary player to get a nest egg going for their character by using the primary professions offered in World of Warcraft.

If you haven't read my previous article on using secondary professions to make money the first 20 levels, you might find it useful.

Keep an eye out for my next article on making gold work for you in WOW where I will discuss different strategies on using the AH to make money, farming for fun and profit, and controlling the market.