Also, try to finish building CS early. A helpful formula I use is:

Number of CS =Sq rt(land goal)/2

It's not exact, but it's a lazy player's prime efficiency formula.

As Gerdler pointed out, you'll also want to try to delay your purchasing military as much as possible to the end of set to maximize your income. People generally buy bushels that they put on the market to avoid decay, and try to sell and spend all the money right at the end.