I never, ever raise taxes above the optimal level for them to upgrade. The populace gets pissed, and leave and then you're in a lot of trouble.
I sell the most valuable items i can from my home warehouse to the roaming merchants (fleets of your trade partners etc). Have a few trade routes that always bring the main product to your main island, where everything is consolidated and used/sold. It's probably not as efficient as rotating resources as they are precisely needed throughout the islands, but that is a side effect of the deficient trade route system. However, the upside is that it at least leaves you able to keep the trade routes centralized so you know that finished products leave the main island, and raw materials come to the island.
Also, I keep building houses. They are the biggest single source of income you have in the game and making sure the population is happy is insanely important in this game, even on easy mode. More population = higher tax revenue, which is what we're after here. There is a lot of micromanagement of the industries you don't need to curb your spending; i generally turn off non vital industries when i must, or keep an eye on my stockpiles of goods and turn off industries I don't need. You really can't maintain a negative income for long, or **** hits the fan very quickly.
I also find it useful to never introduce a new good into my markets (read as my population) without having a substantial amount of it in reserve. For example, I will disable the population's access to Spices when I first get access to Spice, so that once I generate a reasonable stockpile (10-15 depending upon population/length of trade route round trip) I can then enable access and not worry about running out of the goods quickly. Anyways, I use that for all the other major goods that your population needs. Again, might not be the most efficient way, but it is cautious, and caution keeps you financially sound; reckless spending is punished very very quickly
