This achievement isn't too hard. The only thing you need to remember is that every decision you make needs to be one that increases your city's gold production per turn. For example: Instead of settling a Great Person in your city, take the gold they could provide instead. (This example applies only to Great Person(s) that have the option of providing you with gold)
EDIT: A very good point has been made by Wyrmser, there are certain "Great People" that will increase your gold production by some percentage (usually 50%, I believe). I advise that you DO settle those people, rather than just take the gold they provide.