1. Professions Are A Good Start:
Primary and Secondary professions are good places to begin making money from. Most professions allow you to make goods for other characters that you can sell to them.
2. Learn To Use The Auction House (AH):
The AH can be your lifeblood whether you are dealing in potions, epic weapons, or reagents. Learning to use it can be tricky, but is more than worth it when you find a hot commodity.
3. Keep Your Eye On The Trade Channel:
When you can, keep your eyes on channel 2 because occasionally you will find very good deals as many people are competing with the AH in terms of price and this can be even better if a particular market is in a slump.
4. Know What You Are Selling:
This seems to be common sense, but if you look up item A and it has 5 more uses than item B, it's probably going to sell better. If item A is a trade good or consumable, it should be easier to manufacture. The better you get at making Item A, the better the feel for the market you will get.
5. Know Thy Market:
This goes along with knowing what you're selling. If you sell an item long enough, you get a feel for the average price on the server (a good way to know when someone is trying to cheat you), the major players in niche markets, and exactly how much profit margin you can get for your sale.
6. Invest In Macro Training:
If you are planning to bark out that you are doing enchants in a busy city, make sure that you can do so without needing to type a small paragraph each time. 3 lines seems to be the respectable max for a post in the trade channel.
7. Fishing May Be Boring, But Worth It:
You would be suprised at what fisherman pull up. Yes, I know it's boring sometimes, repetitive all the time, and most of the time you get worthless fish. But if you know where to fish, that changes the game. Fish schools of specific fish like Oily Blackmouth or Firefin Snappers and they sell well in a stack. Find yourself with some Stonescale Eels and they sell for gold. Not to mention all the junk you can get out of the water from wreckage.
8. Make Alts Work For You:
An ALT is an alternative character often on the same server. If you have some extra materials that can be processed and resold, send them off to the alt, log in an hour later and run them through the profession of choice (will require leveling in most cases), send them back and resell.
9. Have An Alt In A Capital City:
Since you don't always play Alts, a tip I read about somewhere made the suggestion to have an Alt in a capital city so that your main character doesn't always have to travel back to post item in the AH. Definitely a good idea if your character is going to be away in the wilds for a bit. This works as long as you can easily get to a mailbox.
10. Gold Farmers Are People Too:
This is fast becoming a time honored tradition in WoW that is simultaneously loved and hated for various reasons. If you're good about it, farming from a gathering skill in the right areas is like panning for gold and consistently getting it.
11. Be Nice To People:
Another no brainer, but be sure to be nice and respectful when closing a sale because they are paying you for something that they could get somewhere else. Plus, it might even net you some additional sales down the road and increase your reputation on the server.
12. Don't Be A Ninja:
If you're in a group and you need an item, ask first. If you just want that item, again, ask. If you roll need on a item you can't use or just because, that makes you a ninja, and yes, ninjas don't get re-invited into groups.
13. Hit The Smaller Instances:
If you are a higher level player (45-80), there are lesser instances that you can solo to get additional money. Granted, most items coming from places like ShadowFang Keep and BlackFathom Deeps aren't worth a lot, but keep your eye out for rare items that fetch handsome prices on the AH as players buy them for ALTs.
14. Find Limited Recipes:
Most recipes can be bought from a specific vendor, however some can only be bought once or maybe twice from a vendor in a certain time frame. The recipes that have limited availability and have a vendor who may not be easy to get to will usually sell very well.
15. Secondary Profession Books:
A corollary to the above rule might be to not sell unlimited purchasable items. However, the exception to that rule is that books for advanced Cooking, First Aid, and Fishing tend to sell marginally well and can be usually purchased for a gold a piece. Sell it for 1.5 or 2 gold on the AH and that profit will add up over time. Just be careful not to flood the market with these.
16. Ride Like The Wind:
I know part of the reason for these tips is to be able to purchase a mount, but once you have a mount, that is a tremendous speed boost in your gathering ability that can help you earn money at a faster rate. Plus, it's nice to be able to travel at faster speeds.
17. Neutral Auction Houses
Keep you eye out for factional type stuff that can be sold over the neutral AH for a decent profit. Recipes, pets, items, and more can all be sold for gold if you have the patience.