Well, obviously levelling can generate funds, but when I'm really in the need for some gil, I farm. Silk threads are still selling for 70-80k in Jeuno, and I can usually get an entire stack in under an hour. I tend to have decent luck with that, so I'll farm a couple of stacks and then head out to Jeuno.
I put them in the AH for 65k every time, in an attempt to continue lowering the market. Of course, when I do that, I often get 75k or more for them, simply because not many people think to buy low before matching the current prices. Works for me.
