Trade is underestimated. Running furs and iron from Vaegirland to Rhodokia can be lucrative, as can running spice from Khergistan to Nordholm and velvet from Rhodokia to Vaegirland. Beat up sea raiders and deserters on the way for extra. Sometimes you will actually run towns out of money this way. You just need good trade and better inventory.
After you get your own towns and have guilds built, investment can make the most money. If you time it right, you can get an investment cycle in before payday. That's important, since the most investment money seems to be made when you invest your entire cash reserves. Returns of 150% are not too hard, but you can lose it all. Might pay to have some expensive items in a chest just in case (spice and velvet work well).
There may also be in investment bug. On one occasion I invested about 200,000 denars and got 17,000,000 in return. Invested that again and got 52,000,000. Must be those Imperial junk bonds.
While mercenaries and regiments are quite useful, they are also quite expensive.