Remember to properly develop all of your cities and turn any castles that are not necessary into cities for the economy. Try to not enter war with everyone at once since your trade will just plummet. Also, try to garrison all of your cities with the free upkeep units. Disband any mercenaries that are not being used due to their high upkeep, and in general try to sack every city you conquer since relatively little population is killed while you get a large chunk of money