One at a time, buy the other workshops of the type you want to make money from and convert them to something else (ideally something that won't bankrupt the NPC too quickly) so that you eventually become the sole producer and seller of the good you've cornered.
It's important to note that you'll have to shut down new competition over time, but it's at a fairly slow rate in my current playing and, according to an old video of Strat Gaming's, is based on how long it takes for the NPC to go bankrupt.
It requires touring the continent and noting what is where but the end result is ludicrous amounts of money not just passively earned but actively earned by selling held-onto production for you to pick up at the warehouse. Oil, which I broke into after Velvet, went from producing the typical nothing per day to ~1000 while holding 25% of its product and ~700 while holding 50% of its product once I bought out every other Olive Press and shut them down. It had costed me, I'd guesstimate, 200,000 denars but I made that money back within a single season and have become a millionaire by the end of the following year. I'm thinking of doing it again with Pottery should I keep this going and not enter politics (kinda hard to run a global monopoly as a warlord lol) in the near future.
Caravans have also worked well for me and I suspect they increase Workshop profits by driving up demand since they're participating as consumers in the economy.