Sort your list of provinces by manpower and increase the barracks level. This has to be balanced however as when you increase the barracks level it also increase the amount of food you need to have. If you are producing plenty of food per hour them you can afford to upgrade your barracks in you best manpower cities and leave them on.
Also man based units have a manpower per day cost, don't have any units idle, send them into battle and start balancing your unit loads out with more oil based units. This will free up manpower as well.
"It is even better to act quickly and err than to hesitate until the time of action is past."- Karl Von Clausewitz
increasing province morale also increases manpower production. How higher the barracks level, how higher the MP income. So build a lvl3 barracks. Building a lvl3 is better then having many lvl 1 barrakcs in low mp provinces.
You also might want to focus on low cost MP units. Infantry eat way too much of mp and food. AT/art/AA eat allot too.
Planes cost allot in resources but low MP, same thing for tanks.