You touch on a good point Monsieur Alphonse: diversification of plantations.

It's especially relevant in Indian and American colonies, where often you have a choice of which commodity to produce on each plot (some, i.e. most in the caribean islands, with the exception of Cuba and Santo-Domingo, I think are fixed to sugar).

You will usually have the choice in America of Coffee/Sugar or Tobacco/Cotton and in India of Cotton/Tea. It's a good idea to diversify your supply and not put all your eggs in one basket, so to speak. In other words, develop a proportion of each commodity so that your trade is more robust and (hopefully) more resilient. Remember that Sugar and Spice can be supplied in large quantities by trade fleets.

In line with what Slaists has said on controlling supply and demand, and only because it hasn't been explicitly mentioned yet, it's important be mindful that you are trying to balance the quantity of goods you trade with their price.

What I've yet to be able to project is to what extent small increases in supply and small decreases in price do balance out. The complexity of it lies in all the other factors in trade: port, town, infrastructure & city development of player & AI and the number and worth of trade agreements.

I generally go softly softly though. Pre v1.2 your revenue would jump whenever and however you upgraded plantations, there is a finer balance now in v1.2 so after capturing territory and trade nodes (in the early game, with about 2-3 ships), I don't automaticaly upgrade them or add more ships until the prices actually start to increase.

Price commands supply for me, but rest assured that even if you do over-do the supply a bit, the prices will always recover.