I do fine increasing my army income per second in the beginning of the game and then my ability to increase the army income slows down a lot. I spend a ton of W money to improve the army camps but it only increases the my army income per second very little. Am I doing something wrong, is there something else I can do beside capture Army Camps or supercharge an army camp by spending Warzone coins?

You should aim to get the techs as soon as possible, so you can have a little boost for them. On top of that, spend your AP on army saving upgrades, like Joint Strike (max this to 25% asap).

Also upgrading hospitals is a great way to save more armies. Saving armies is important, cause every army you save is an army you dont have to produce with your camps.

Thanks.