Another thing not mentioned. Make sure you have the Challenge Log unlocked. The unlock quest is level 15 in Limsa, so you might have done it ages ago. It rewards you for playing the game with a lot of weekly-resetting goals. Are you doing something in the gold saucer? It will reward extra MGP. Are you crafting? Here's some bonus exp. Are you playing PvP? Extra wolf marks or PvP exp. And above all, more gil. Any task that doesnt have an immediately obvious reward will offer gil instead. All small amounts(1k-15k), but each is available weekly and it just ends up being a bonus for doing what you were always going to do already.

Beyond that, ill second hunts and maps. If you have the time to do every daily hunt mark it adds up to over 100k per day(time investment varies with mobs randomly selected). Thats in addition to the currencies you can use like Jojoya said. Thats in addition to any trains you might get in on. And a good map party can easily make hundreds of thousands of gil per hour. Again, not counting getting lucky on any rare drops you can make extra for.

And final note. If you have any resources that cap do your best not to let them. Poetics? Spend them before they hit cap. If you dont need gear, find a way to turn them into money. Either via GC tradeins, selling crafting mats, or whatever else. GC seals close to cap? Buy something to sell/use. Other then MGP, every special currency either has something you can sell, something you can use to save other currencies(ventures, aetheryte tickets), or something you can bring to your GC to use THEIR currency instead.