You thrive on the income of your warfare as your spoils of war. However you need to invest those in industry in your urban and rural core provinces. If you use your spoils to build more and more troops you will get in trouble as each unit has a daily upkeep which weighs down your economy.

To improve your regular game play and which units to use I would refer you to the community written guides section.

Ceep your production under controle, most of the players make mess out of their production in mid late game. Upgrade your units production buildings on max and have 1 city for 1 unit only. Dont build artilery and tanks in same city cause you should have enof lf urban province for healthy production.
If you are very wealthy on resurses you should start upgradeing all airstrips on max, if you embark and disembark alot make docks lvl 3 and start with some infrastucture to move your units arraond faster.

Don't be like this noob (who actually has a forum account but shall not be mentioned

Always focus on building your Core provinces with Industry and Manpower and Unit building capabilities. There comes a point where you capture cities and begin to focus on improving Unit production there. Then you can build units closer to the 'frontlines'. A level 2 Ordnance Foundry in an enemy city builds a Unit just like the one in your Core - except for the effect of moral. Use the Propaganda Office to help boost moral and get production moving faster. You Core Foundry may produce a Unit faster but if it takes longer to get to the frontlines from there then it is something you must consider.

At mid late game, my industry is allways maxed out, or good enof for my needs. If you dont have industry in mid late game, you wont make it mutch further then that.