Buy supplies from villages and ferry them to larger towns. You can also buy cattle, slaughter them right outside a city's walls, then sell the beef for a pretty fair price. This can be especially lucrative if you're a neutral character that won't be jumped by aggravated lords.
While you're at it, watch out for any parties of rebels, deserters, brigands and looters. You get a small amount of thaler from winning each battle (and also a small renown boost), and you can sell the captured weapons and armor at towns.
I also usually like to pick a faction to beat on. Usually just one, a faction which is usually kind of wealthy, and raid their caravans. That can not only help boost your renown considerably, but taking the goods from the caravan and selling them can also help give you some money.
Keep an eye out for quests, too. Debt-collection ones are kind of boring, but they're handy early game because you can use them essentially as small loans with no interest. Use them to buy better equipment and hire better soldiers, then go looking for brigands or caravans, etc.