"Wait in city" -- make 7-10k a day
Can confirm that if you build a workshop in a city, and then whatever kingdom you are a part of declares war, they SHUT DOWN THAT WORKSHOP. Build your workshops either in your own kingdom (under a different lord or your own, although I prefer my own because I will go to war with my previous kingdom after I leave them). As for making money outside of war-time. I haven't checked the numbers on this, but to me it seems like a unit is 'worth less' if it's in a garrison than if it is actively traveling with you. If you have any fiefs, dump a lot of troops in there and run around with a smaller army. -- I haven't confirmed if this is the truth, please do check and report back.
As for actually increasing your income, I was sitting on about 500k and losing denars by the thousands per day. I got all my companions together and set
every last one of them off with a caravan, supposedly making them a party and having them run around makes you more money, but I noticed that it's usually very high maintenance, they'll get captured by something, then they'll not be able to build up an army without you holding their hand.
All these caravans together got me into positive on some days, although it was jumping between -1000 to +500. Not enough. (At this point I already had the max amount of workshops too). So I started looking at my money tab, seeing what costs I can cut. I noticed I had quite a few mercenary groups under my rule (Own Kingdom), fired them and had +4000/day average. Now, I assume this will not be the case for you (seen as you're part of a kingdom), the way I did this while I still fought for the Khuzait was I'd run around capturing every lord with a 100% cav/horse archer group, (allowing me to take them on even if they outnumbered me 5 to 1), then selling their loot and ransoming the lords, obviously that is during wartime, however there is NOTHING stopping you from declaring war, just run up to a lord "I have demands, suck my ****" and **** them up.
If you don't wanna do that, just running around killing bandits works fine too.