The thing about the trade rights can't be stated enough. Send out diplomats, secure trade rights with everybody. Don't go to war with more than one or two factions at once, so your trade doesn't get hit too hard.
Remember to clear rebels away from your roads once in a while, too: These block trade flowing on that road. Attacking with a superior stack drives them from the roads; let them sit in the forest and rot, as long as they're not hurting your money.
Also, don't underestimate merchants. Level them up by monopolizing some simple resource close by, then send them abroad. Depending on where you are, try to find the following: sugar in southern Spain; amber in Norway; gold and ivory near Timbuktu; silks near Constantinople. I had typical exp eight merchants rake in more than 150 a turn, sometimes far more than this, up to 300. That way, merchants pay for their rather high starting cost and provide upkeep for one elite unit. Granted, you'd need a lot of merchants to pay for your armies, but every little helps.
Keep your ports unblockaded by maintaining a good fleet. This can then also be used to blockade enemy's ports to reduce the armies available to them a little.

I have to restate one point over all the others, because it's so important: Sack cities and castles when you take them! This nets you a five-digit sum typically, enough to keep you going for a long time. Even a full stack of elite units costs only around 3000 fl / turn in upkeep, so the 15000 or so you can get out of sacked settlements really help you take off.