If you have many family members and generals, instead of having them concentrated in one area, or sending them all off to war, have them govern your cities and castles. This will reduce your need to have more units in certain cities or castles since governors are a huge help in keeping public order.

I also suggest you keep your specialists limited. Though assassins, spies, and diplomats can be useful, they need money for upkeep. And don't forget to keep your provinces in good religious order. Religious unrest can cause devastation.

Don't build too many navies or ships. I keep one navy, of about four to six ships for every four to six ports.

Use your diplomats and your princesses. Establish trade and relationships with other nations.