From a variety of ways really, having all professions maxed (although in my case not all as i don't have lw or engineering maxed, will do soon so my earnings will go up) does help as it means you aren't dependant on anyone else for use of cooldowns or crafting etc.
I just branched out recently a bit more on my shaman alt into pot making since the market for flasks returns little profit for the cost of mats, whereas potions do, crafted 1200 potions 2 days ago and will shift them over the next week or two, and that will return 3-4k profit, along with other things.
I haven't bothered with inscription/ glyph selling for a while as work prevented me from updating my auctions often enough, but i used to make akilling on glyphs, at least 1-2k pay day.
Other things sell suprisingly well sometimes too, fortune cards go through random phases of peaking and troughing, when the demand peaks i sell them as fortune cards, when it troughs i sell them as fortune cookies.
I also sometimes try to shift transmog gear, sometimes it sells sometimes it doesn't had an enchanted thorium breastplate (quite rare on some realms) at on ah for two weeks now with not even as much as a bid.
Ah flipping is another thing that can generate a lot of gold, boe's epics, and other things when people undercut too much buyout their auctions and relist.
I do have othe gold on other characters that i use as bank alts or auction characters. just snapped up quite a few epic gems at about .6-.7% of market price, so when i sell them all that will return a tidy profit, even if it takes a couple of weeks.
The main problem people seem to have when they sell thing's is they either over or underestimate the value of their items, competition and selling timescales.
It's no use listing your pots or flasks at 4pm sunday, as many guilds don't raid those days and prices will often bottom out, meaning people will buy your auctions and resell them for profit at peak, when you could have made yourself more profit by selling them during peak
Eu.theunderminejournal.com
Use that as a base to analyse your market and when your competition places their auctions, strategise accordingly