How to get the money

[Philbert]

Every now and then I go through my list of settlements to see which settlements are content (green face) to see if I can extort more money out of them (by nudging up the taxes). This can make a huge difference.

[homefry]

Keep your army upkeep under your tax income. This is a good gauge to tell you how many more units to recruit.

Plan ahead. What I mean is you don't have to always be building in settlements that will probably become a castle. (example... Byzantine... there is no use building infantry buidlings in castles in the early game because your cities produce the same spearman.

Going off of Byzantine example. You have to go easy on building buildings with this faction because you start out with low money and have more settlements than other factions and Constantinople is already an advanced city which requires expensive buildings.

Quickly recuit merchants and grab all 4 resources near constantinple (cotton?). Your merchants will level up quickly and as soon as 1 of them gets 4 points, head west for the gold lot near Ragusa. There are always tons of enemy merchants in that vicinity and your high level merchant has some easy pickins.

[Zenicetus]

Adding to what the others said:

Always build a road right away in a newly captured province for the trade boost, if there isn't already one there. You don't have to build the road upgrades until later, but that first one is important. Upgrade to paved roads when you have a little extra cash and don't need it for something else.

Always build a port as soon as you can, in a coastal province. Following the line of port upgrades is a great way to boost the economy.

Be frugal with use of mercenaries, since they tend to have high upkeep. I'll frequently hire a few mercs if I need the extra manpower to siege a settlement or win a field battle, and then disband them right after the battle. That way it just becomes an added one-time price to capture the settlement, not a long-term drain on resources when I could build a similar unit with lower upkeep costs (and easier to retrain, most places).

In a similar vein, don't keep depleted units around that aren't militarily effective. Combine depleted units after a battle. If you're left with a few very low-strength units, either move them to a settlement where they can be retrained (if you can't do it locally) or disband the unit. Don't continually pay upkeep on units that aren't useful.

For city garrisons, make sure you use free upkeep Town Militia instead of other units as the standing garrison. Don't use more garrison than you need in castles, since morale is high there. Just a few peasants should be enough.

I tend not to use merchants at all, unless I have easy access to Timbuktu. Too much micro-management for me, in leveling up and defending against hostile takeovers. For the way I play, I've found the real key to a healthy economy is managing the upkeep costs of the armies. I try not to build a single unit more than I really need to maintain homeland defense and whatever conquering armies I'm running. Cutting the armies "close to the bone" also makes the game more challenging, since I'm not always steamrolling the enemy with superior numbers.