Also:
when you just want to increase the speed of production, but not neceserily produce in the next turn, dont rush it all the way to the end.
For example, you are heavilly in war and producing knights. You have a good gold income that you want to spend on increasing the speed of producing knights.
Of course as said, either wait until you have some shields produced, or disband a unit or first rush a worker. But not rush a knight when you have 0 shields in the city.
But if you rush the second half of the knight, you are still wasting shields since you buy all the shields to complete it and the knight will only be complete the next turn. In that turn shields could be produced, but they wont be used anymore.
Therefore:
If you rush a knight but you have different cities to choose where you could do so, rush it in the city with lowest production.
If you don't need to have it next turn, rush a unit somewhat cheaper than a knight and rush that. Then swich back to knight. It would be ideal if the difference is cost (the amount still needed after doing this) is the same as the cities production or only very slightly lower. You can also choose to have 2 more turns of production. For example, if the citie has production 15, first make sure you have some shields in the city, either by producing them, by rushing a worker or by simply disbanding an obsolete warrior. Then rush a medieval infantry for 40 shields so that you have 30 shields remaing for the knight. The knight will now be ready in 2 turns and the cost relatively low. Nothing is wasted.
What i also do, even when i have no money to spend is shown in my next example:
you are producing a knight (70 shields) in a city with 17 production. If you do nothing, it will take 5 turns and you will have 85 shields produced. 15 shields are wasted !!!! So after the first turn, when there are 17 shields produced, i rush a spearmen for only 12 gold (3 missign shields x 4 gold per shield). Then swich back to knights and produce the remaining 50 in 3 turns. This was, only 1 shields is wasted rather than 15 and 1 turn is saved for the mere price of 12 gold.
That does require some thinking. Probably too much for most casual players, but it sure helps if you want to reach a higher difficulty level. (Or if you are playing gotm and want to get your victory as early as possible

)