TL,DR version:
Farms, ports and regional special resources
Field ashigaru heavy armies, disband land and navy units not in use
Castles as low as you can get away with, though garrison requires no upkeep and offers repression bonus, so a few levels will save you money in the long run
Special production centres and good roads + encampments = better than every province trying to tech and pump units
Trade ships, trading rights - don't be afraid to bribe factions to sweeten the deal, you'll break even after a few turns
Invest in your 'money centres'