i'm not sure if this strategy has been mentioned before but it's great . OKay, you build a strong culture with your civilization and leave a small spot for another civ to come on your island. After a while, they will be assimilated through culture. then, sell that city back to the original civ and they will give you( from my experience), world map, practically all their gold, 40 gold per turn, and maybe a resource too! This is great.... then after a while.. the city will be re-assimilated... just repeat this process and you will be milking that civ!