Here is some explanation for the advice above.
The fundamental resource in the game is turns (also called months), and your job is to use your turns as efficiently as possible.
Early on, you can only pray that you get such a nice setup that your income will quickly outstrip your desire to spend money, otherwise you'll find yourself sitting around for many months waiting for your next annual paycheck (this kills the score).
By the midgame, you should have a decent income established, and many places to expand to, so you should expand.
By the late game, your empire will fill most of the board, and the remaining places to expand will be unappealing. However, you'll have technologies like infratech and trade hubs that enable you to develop your existing empire more completely. You should take advantage of these technologies to make your people as happy as possible before the end of the game.