-Offer a trade to another civilization (a technology or resource they do not have) for an incredible amount of money. For example, coal for 2 followed by as many zeros as can fit. Do this once per turn.
-You must be at war with two people to do this trick. When you can propose peace to either opponent, refuse twice to the peace treaty. Then ask, "We would like to propose a deal,". Ask for everything they have including lump sum, then ask for 9,999,999 gold per turn. You will own the cities and the maps to their civilization and the items you asked for. You will not get their fortified units in their cities, and the capitol city. Also, you will lose all of your money at the end of the turn, then regain it the next turn.
-When trading with the CPU, ask for 11111111 gold per turn. In exchange offer him anything (even 1 gold or your territory map). You can also ask for technologies and gold when offering the same lump sum of 1 gold. With the money you get, you can buy buildings (when your civilization is republic or democracy) or even all the CPU's cities. After the twenty turns your money disappears, but you can always ask for more.
-After playing for some time and you have met other civilizations, start communication with them and ask for 1 gold for 20 turns) and give them 18 gold (lump sum). When the other civilizations will get richer, ask for more money (for example, 10 gold per turn and you will give them 180 gold or more in a lump sum). You will get more money each turn and you can give more money to research.
-Choose diplomacy with a neighbor and offer them a resource that they do not have. Ask them for 200,000 gold per turn and they will accept. You can ask them for other things (cities, techs, resources), but try leaving a couple cities with them so they can pay you so many gold each turn. A few turns later you will have millions of gold. Command one of your units to pillage the road on the resource you are giving to them so that the trade will be canceled. They will not give any gold per turn, but you can safely keep the other gold.