Invest your gold for upgrading your most effective hero and or villain before you change your armies. This way the non-active army will still earn coins for you even if you do not use them. note that the more effective your army is, the more coins they will earn for you, even when you are offline. One thing more, your hero's damage will increase by a portion of the overall DPS of all of your other heroes and bad guys, so upgrading another hero will also upgrade your main hero.