In most cases the biggest obstacle in one's gold making plans is the lack of time. Everybody has a life, and there isn't enough time to do everything you want in the game. People learn to prioritize, doing only the most rewarding businesses, but how you find those is a real challange. It is not simply just about calculating gold per hour with a spreadsheet, there are more factors in play.
If you have many professions maxed out, crafting is most likely your biggest source of income. It is also the most time consuming. However different professions require different amount of time, based on which you can prioritize them. The most time consuming of these activites are, of course the shuffles. Prospecting ores and milling herbes can take hours and hours of clicking on the same button. They are very rewarding, but if you have limited time, it is best to put it off for some time. After the shuffles comes the products that are low on price/unit (even though the profit margin can be very high). I'm talking about items that you are selling in massive amounts. Mysterious Fortune Cards, inks, glyphs, Hypnotic Dust, etc. They are great, but crafting an item usually takes about the same time (2-3 seconds), and to make the same profit as you would by selling 1 piece of bloodthirsty gear, you would have to make about a hundred mysterious cards, which would take much more time. The last part of the business to shut down is the high price items. Gears, expensive enchants, decks, etc.
Reposting on the AH, can take the most time, however it is necessary to make any money, so you can't put that off. However there are ways to optimize it.
- Start a bank character, and load it up with big bags. Bigger bags means less rounds at the AH for reposting.
- Start posting items for longer times. 24/48h.
- Level your character just enough so you can get the fast flying mount (or make one of your alts your bank character). In some cities like Orgrimmar, there is quite a bit of distance between the bank and the AH. If you are storing items in your bank/guild bank, and your business is so big that you have to make about 6-10 rounds from the bank to the AH, flying over instead of walking can save you 2-3 minutes in total. Notice that you can mount while you auto-loot the mailbox, so while you are getting items that are expired, or you cancelled them, you can mount.
- Use /reload ui macro. While you are auto-looting the mailbox, only the first 50 mails are displayed, and they can only be refreshed every 60 seconds. That is if you don't reload your UI. If you have a fast computer and internet, looting that 50 items will only take you about 30-40 seconds. If you reload, you can hit auto-loot in 5 seconds instead of waiting 30. In total this could also save you 4-5 minutes.
- If you haven't restocked your products for a while on either factions (in case you are doing cross-faction trading), you will end up with the most unwanted items on both sides. For example after about 2-3 weeks, everything worthy of any decent amount of gold is sold, so in the end you will make only 2-3k of gold daily, even though you still have 500-1000 items rotated on the AH. In this case it is wise to swap your stocks of products on the two factions. It takes less time then acquiring materials, and recrafting everything, and the products that are not wanted or low on price on one faction, could be great on the other. Using this method, I boosted my income from my burnt out stock from 2-3k/day to 12-13k/day.