Reducing "Speed" by even 2 knots on a modified warship design yields a lot of room to add more powerful weapons and/or armor. This is especially true for CA's and CL's. By stepping them down to 30 knots and reducing "Scout Planes" to 2 creates additional capacity for armament with very little loss of battle performance, but a far deadlier ship in combat.
Do not hesitate to "steal" other nation's new ships that are available to build. Look for the highest steel limit in a class (for example, BB's Yamato-class), and copy it to your navy for development. By selecting the one with the highest displacement to steal and use, there is greater room for additional armament, aircraft, or armor. If you are not playing the U.S. and are into carriersm wait for them to develop the "Midway-class" and steal the design. The same thing also works with aircraft.
Always keep at least one Fleet open for new ships at the Home Port. Individual warships cannot be sent off to join a Fleet alone, and without an open "slot", they remain in the Shipyard, not in the war.
As any nation, immediately rebuild your best Heavy Cruiser class to include as much torpedo armament as possible. As soon as you receive "Middle Diesel Engine 1", upgrade that Heavy Cruiser with a higher Torpedo armament. The best you can get is "Quintuple" with "10 Bay". These will prove devastating in ship vs. ship battles while retaining their bombardment abilities.
When playing as the Americans you will need the following to defeat other fleets: fifteen battleships and only a few aircraft carriers. Also, only attack at night.