The simplest way would to buy crowns and exchange them for gold at Zeke, but if you can't, then try these tips:
1.) Play mini-games instead of catching air wisps, you heal while you play the game, and at the end you get mana and gold too, maybe a treasure card or two.
2.) Sell things you drop, like while I was saving for my house, I had one of everything: One hat, one robes, one shoes, one wand, etc. Also I sold all my furniture that wasn't a quest or picked-up item.
3.) Make a daily budget and stick to it. Example, you need some treasure cards but you're saving for a house, so your budget is 300 gold a day, so you buy 2 treasures that cost 150 each, and you don't buy anything for the rest of the day. Heck, you can make it 300 or something a week if you want! The key is to not urge yourself to flow over your budget. It's okay if it's, say, about 5 gold over the budget, but every gold counts.