Not so on another realm where you don't have a main, and I realized that unless I went back to a few monetary basics, my alts would wind up dancing naked on mailboxes in pursuit of gold. This is a fine tactic with a long and storied history, but when your most promising alt is a level 16 Undead Mage, you're up the proverbial creek. No one wants to see a rotting, naked corpse.
So I started not being a deadbeat, and it was with surprise and delight that I logged on to find the little tyke sitting on a pretty respectable pile of gold by level 21 -- as in, he can afford to pay for his level 30 mount and training several times over, and still have enough left over to train himself all the way to 45 even if he doesn't make another penny.
I should note that this assumes: a). You don't have a ton of time to spend on alts, b). You also don't have the gold pool necessary to play the AH like a tycoon, and c).You want to make a decent amount of money without having to turn it into an overriding concern.
1. Sell absolutely everything.
You really never know what people are looking to buy at any given moment. Raptor eggs? Lion meat? Wool cloth (a surprisingly lucrative item, for whatever reason)? Solid stone? Volatile rum? A wealthy player who's spent a lot of time at 70-80 with months of dailies won't bat an eye at shelling out a few gold for these items; the convenience of grabbing them off the AH vastly outweighs the lengthy trips they'd need to make otherwise. And with so many people leveling Death Knights, there's a huge market for low-level profession materials, especially Cooking.
If it's gray, vendor it. Everything else, sock it in your bags and hit the AH. You probably won't make a huge profit on each white item, but the little stuff starts to add up.
2. Know the server.
How large is the raiding pool? Is there an active RP community? Did a lot of people wind up rerolling Death Knights, or leveling them seriously? Are people willing to spend a lot of gold on items for Achievements?
All of these affect what your server's common market is going to be like. Raiders spend a lot of gold on consumables and/or materials (which you really won't be able to do anything about before Northrend, but it's incentive to keep going). RP servers tend to create a better market for event-centric items like fireworks or things like Dalaran Wizard's Robes. Large groups of leveling Death Knights gobble up runecloth to improve reputation with home factions and go through a ton of profession materials.
3. There's still no substitute for two gathering professions -- and if you're serious about it, use a mod like Gatherer.
When you're starting on a new realm with no resources beyond a strong work ethic, the amount of gold you'll have at your disposal is likely to be minimal, and not enough to allow you to use Auctioneer to buy low and sell high. With time and some effort you'll reach that point, but in the meantime, don't cripple your cash flow by taking anything other than Skinning, Mining, or Herbalism as professions.
Herbalism feeds Alchemy and Inscription now, but Mining still feeds Blacksmithing, Engineering, and Jewelcrafting. The financial gulf between them is considerably smaller than it was during BC, but mining probably still retains the edge as the most lucrative gathering profession overall. With that said, it's also more competitive than herbalism, especially as you near the later levels. If you're on a very crowded server, keep that in mind.
Skinning is by far the easiest and least time-consuming profession to level. You'll generally make less selling leather and hides than you will selling herbs and ore (at least in early levels), but you won't have to go out of your way to get them either. If you take herbalism and mining, you'll also have to spend a lot of time flipping between two tracking modes to see what's available in your area. If you want to make money while devoting as little time to it as possible, skinning's the way to go.
Thunder Bluff is by far the best overall with respect to flight-path proximity to auction NPC's, the bank, and a mailbox. It also gets bonus points for having a forge literally in front of auction NPC's if you're a miner and selling bars is more lucrative than selling raw ore. Undercity, Ironforge, Stormwind, and Orgrimmar are all OK (unless you have to smelt, in which case UC, Org, and Stormwind are all a much bigger hassle than Ironforge) but generally more convenient if you're already got a bank alt parked there.
Silvermoon and the Exodar are both lousy places to do business, especially if you're flying or running into the city to get auctions done. They're OK for bank alts, but in general you're still better off putting that alt elsewhere.
6. Don't be in too much of a hurry to level.
With the boost to leveling speed as of patch 3.0.2, clocking off the levels isn't anywhere near as time-consuming as it used to be. This is good in some ways -- spending less time leveling a character is great, and an ideal situation for someone who already has a toon at 80 -- but it has the secondary effect of being harder on a new player or a reroll who finds the game's most pressing expenses -- principally mounts -- coming earlier and faster than they used to.
It's not a race. There is no law in the game dictating that you must have a mount by the prescribed time. Take time out to gather, or grind on mobs that have a chance to drop something fun or lucrative (especially dragon whelps that drop pets or, say, Barrens mobs that have a chance to drop Recipe: Savory Deviate Delight), or even just fish. Spending more time per zone doing fun quests and enjoying the lore just increases the chance that you'll get a valuable green or blue drop anyway. Turn money-making into a game within the game itself.
The reason for this is that most ingame costs are static. Sharp arrows will always cost the same amount, reagents will always cost the same amount, food will always cost the same amount, and the only thing that affects them is your reputation with the faction of the NPC seller. The same is true of ground mounts in classic WoW. A server's inflation increases the amount of gold you can make off of the AH, but your daily living expenses will always stay the same.
8. Cannibalize otherwise useless characters that you know you're not going to level.
Even a fairly low-level character can be broken down into a few gold if you're desperate for money and you know perfectly well you're not going to wind up leveling it. If you've got a character below level 20 that you really don't need, sell off their assets and gear and mail the proceeds to the toon you really want to level. If they're above 20, they'll usually wind up being worth more money, but be very cautious before you write off a toon in which you've invested more time. It sucks to have to come back and re-level a class that you've realized you do want to play, especially when certain classes are more time-consuming to level than others.
9. On that note, cannibalizing a Death Knight is a surprisingly lucrative racket.
Pretty soon you'll be able to level a Death Knight on any server as long as you've got a level 55 character somewhere. Death Knights typically finish their starting area with around 30-40g, which is a nice chunk of change for a low-level alt. As an added bonus, leveling a Death Knight from 55 to 58 is fast, easy, and a lot of fun. I really don't suggest chain-leveling and then deleting Death Knights just for the money -- your gold per hour is likely to be considerably less than it would be if you just kept leveling the needy alt in question to more lucrative gathering materials, and this is a great way to make Death Knights boring fast -- but as a one-time thing it won't hurt.
Of the Eagle, Of the Bear, Of the Tiger, Of the Falcon, and Of the Monkey = Sells, and typically sells well.
Of the Owl, Of the Boar, and Of the Wolf = Sometimes sells, and usually for less than the first three.
Of the Whale = Just vendor the damn thing.
I've run across a surprising number of people who just vendor greens no matter what kind they are. Certain ones have stats that are very desirable to leveling characters trying to cover gaps left by spotty quest rewards. Don't toss away a potential 4-5g thinking that all greens are trash!