Hello,
i've been a leeacher for quite long time. So i've decided to share with some methods of gaining lots of gold. May be, some of my methods already have been posted here, however, i hope you won't blame on me a lot, and will find something usefull here 
I make gold using:
1) Herbalism
2) Inscription
3) Raids
1-3) Auction house ofc 
------------- 1. Herbalism --------------
For this you need to have and 450 herbalism, and epic flight mount or beeing a druid (best of all for herb farming).
I spend about 1-2hrs a day farming herbs. By that time i have about 20 Frost Lotuses and a lots of Icethorn and Lichbloom. After putting all this stuff to AH you will get about 1500-2000 gold (depending on server).
Best time to farm herbs is a night time or early morning. I'm using Gatherer (link) and Routes (Routes : WoWInterface Downloads : Map Mods, coords/compasses addons. I think u know what is Gatherer, so there is no need of describing it. Talking about Routes addon... this addon allow you to draw lines on the worldmap linking nodes together into an efficient farming route. Also this lines will be shown on the minimap. This lines really help u to beeing on the right route after some time of farming (helps not to loose your concentration).
This is how it looks:

This are the routes shown on my minimap (by the way, i recomend you to use some minimap addon that have feature to inscease it's scale. I make my minimap about 3x-5x times bigger than it should be. It helps to find herbs without effort):

Also i have some herb spawns in Ice Crown, but i don't like farming there cuz many herbs there require you to kill some mobs firstly:

------------- 2. Inscription --------------
Ok, next one. I have 426 inscription ATM. So, after you got your money from herb selling, go to auction house and buy all avaible Adder's Tongue (Adder's Tongue - Item - World of Warcraft) (as for me, i don't spend more than 1000g a day on it). The normal price for this herbs are 20-25g a stack. After you have all your Adder's Tongue in your bag simply mille it (this is a bit annoying part).
You will have a lots of Azure Pigment and propper ammount of Icy Pigment. Make inks (Sea and Snowfall ofc). After that, go and buy some Eternal Life (it's normal price is 120-150 gold a stack). If you used my herbalism guide, you will probably have lots of Eternal Lifes from herbs.
P.S. don't forget that Inks of Sea can be traded to Inks of Snowfall at Dalaran.
Well, you know what's next
Make Darkmoon Card of the North (Darkmoon Card of the North - Thottbot: World of Warcraft). I make arround 6-10 cards a day. If i'm lucky, 2-3 of them will be one of Noble deck cards, which can be sold up to 1000g per card, and arround 200-500g for cards from other decks.
Also, you can keep some of cards and make a deck for yourself.
------------- 3. Raids --------------
Ok, this is little guide, that will help u to get extra 3000g a week.
I'm a member of quite big guild, and we have weekly 25 ppl raids to Naxxramas. We are using a DKP loot method.
So the thing is that i'm playing as a holy paladin, and in Naxxramas some of items looted from the boss are BoE (bind of equip). All you need to do is to look at the boss drop, and look for BoE items. The tip is, because i'm a class that can be a tank, a dps or a healer, i can roll (make a DKP bid) on this items saying that i'm taking them on the offspec (tanking item as example). I think you know what to do next
Take the item, send it to your alt char, put it on AH, wait for gold 
Well, actualy i think that this will work only for some guilds. We don't put BoE items to out Guild Bank, but some guilds does.
In my opinion, best classes for that are paladins, druids and DK's.
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Well, that's all. I really hope u will like it and will find someting usefull.
Cheers