You can also change one or more of your cities' production to 'wealth.'
In the long run, changing a city's production to wealth is undesirable (at the very least, it ought to be strengthening the military, especially if there is nothing else to produce).
But in the short run, a city producing wealth can be extremely helpful. For example, let's say the AI is a bit ahead of you in advances, and that the AI has begun to build a key wonder, such as JS Bach's Cathedral. You, OTOH, have a better shield city in which to more quickly produce the wonder (and/or you've got a ringer city with a jillion Palace shields accumulated), but you don't have Music Theory yet researched (and can't trade for it before it's too late). How to catch up?
Ratchet up your science slider by turning some of your cities' production to wealth. If your military is ok, then you can sacrifice a little domestic/military production for wealth in order to catch up your science and slam home the wonder.