OK, the possibly most covered topic in WoW ever since, like, July. So I'm going to take a different tack. I work by day in a banking/investment firm (Boo hiss you say, I'm sure, but hey, I'm not biting the hand that feeds). One thing I've noticed, money makes money, so I guess gold makes gold. If there's something on the AH that can be sold for more, chances are it's already quite expensive. Do you have 12.5K to buy chopper parts? If so, I'm sure you'd sell the chopper for 15K, right? Not possible if you don't have the initial outlay, or a very understanding guild. So let's take this to Cataclysm. You're 80, wandering around the new zones, you can't fly in Azeroth until you purchase the license, so therefore, if you're a gatherer you're a step behind the ones that did get it. This means you miss the crest of the wave. When TBC hit, new materials cost a king's ransom, but undercutting worked in the buyers' favour and prices normalised at a high point, then eventually the true prices became commonplace. If you want to take advantage of other people's gold reserves you need to supply their needs before another seller does. Remember the old maxim: If you don't look after your customers, someone else will.
So, my first gold making tip for cataclysm is something of an oxymoron - Make Gold in Wrath and stockpile it. You'll need it to get your flight license, train up gathering skills and simply for stamina. Do you want to have to go back to the Argent Ren faire for gold while others are cruising around new and knackered Azeroth mining all the nodes? Didn't think so, either.
Second tip: Go get things now you can't have in Wrath - The Razzashi Raptor pet, for example. There will be a number of things discontinued by the Cata update 4.0.3, so go read Zuggy's brilliant tips and other sites that list what's going away. Thanks to Breanni for his brilliant site on pets.
Third Tip: Pets - People pay damn good money for them. The Pet Mini Tyreal is going for $985.00(!!!) on eBay. I'm not suggesting you break Blizzard rules, but look at that in gold terms, people will pay good gold for pets in game. With a little travelling (And you can use this time for some archaeology, and XP from travelling - my favourite thing ever - lol) you can amass a decent collection of pets and flog them on your faction's AH for a tidy profit. Part two of this is to organise with a friend to sell pets, roll an opposing faction toon, and visit the independent AH and sell Alliance pets to Hordies and Horde pets to Alliance bods. The secret is to have your friend on Ventrilo or Mumble or somesuch similar. You then sell your friend a grey item for 150G, he uses this to buy his faction pets, he puts these on the independent AH for a tiny amount (With you there on Vent, Mumble etc ready to snap them up before someone else does. You then sell them on your Faction AH, and reverse the process, dragonhawk hatchlings were very popular at start of TBC. Your hordie can farm Black Cats from Hillsbrad Fields. To sum it up: I sold moths for a tidy sum from the Exodar AH, the buyer was a Draenei in the Exodar. This was exploiting one great virtue of WoW players, sloth. (I know it's a sin, but if you're selling to a sloth, it's a virtue)
Fourth Tip: Buy the stuff levellers will need when it becomes cheaper in WoW. When the new ores and cloths and leather hit, the old ones will probably nose dive. People slightly behind the curve (And remember, new expansion means new players, not everyone will be 80 on launch day) will need these things that no one is gathering to level. You could always go gather these things and corner the market, too. Case in point - Wool Cloth. End of the argent ren faire cycle, 5th alt to 80, bored of questing for rep, go to the AH, spend silly money on Wool cloth and get a rep hike with the NPC's outside the tailor trainers in the capitals. Wool Cloth is easy to get for an 80, as in every cloth, ore and leather.
Fifth Tip: On the subject of ore and leather, if you have the slots, or are planning to level a Goblin or Worgen or other alt, roll two chars. Get one to do Mining, First Aid, Skinning, Fishing, Archaeology, and Cooking. Get the other to do Herbalism, First Aid, Skinning, Archaeology, Fishing and Cooking. The reasons are thus: Mining and Herbalism are on the same minimap. This means you can't use the minimap to locate both herbs and mining nodes. The rest of the professions are to gather needed materials, mainly to sell, but some to level your alt and gain access to better materials to sell, level and so on and so forth.
Sixth Tip: (Although you've probably already done this already) Form a Bank Guild. Roll a toon (one of the above ones), go to your nearest capital, and get a guild charter with signatures (You'll probably need to pay people to sign as un-guilded toons are something of a rarity, and they're all trying to make gold too) and get one of your toons to supply your bank toon with bags and gold. Buy all the slots in the guild bank and bags slots in your toon's bank. Use this to store your crafting mats, selling mats, BoE drops for auctioning etc. etc. You can post all your greys to this toon too, because if you leave them in a city, they can vendor all the vendor trash, and use the money for auction fees for the other (decent) stuff you send them. One fun thing to do with Bank toons (And why I recommend Warrior alts as bank toons) is to clad them in interesting garb (I love my warrior wearing the Unadorned Chainmail from the Belf starter area, looks really cool on a female dwarf with a red plait, which is another reason to get a friend to do the honours with an opposing faction toon)
All in all, keep your eyes open, save your gold and spend it wisely. And of course, enjoy :D