Spies are amazing, USE SPIES!!
Its in portuguese but basically i have been intercepting the messages from a coalition for 3 days in a row, caught them early planning to invade me and prepared for it, im playing solo, the rest of the map are 3 coalitions of 3 players, caught them anouncing the day of the invasion wich im very prepared for.
Also managed an agreement from the norh coalition to attack them on their flank once the attack start.
Also hungary and italy have serious problems of iron (detected all their resources with my spies) and put 2 or 3 on their only 2 iron provinces.
All i can say is USE SPIES, it adds a lot of fun in the game and im feeling like james bond :)