what i do every few turns, is check what cities can do with a smaller garrison, and remove just enough so that they dont revolt.

i find that can save you at least several thousand.

also:
make sure that, in all of your cities, your economy is growing. economic growth tends to depend on your upper-class tax. you always want it to be positive, because you will see a steady grwth of income.
in fact, in my current Prussian campaign-in 1751- I have been seeing a rise in about 800-1400 dollars per turn. this is because my economy is growing, and i ave tons of factories.

also: for a less immediate, but still huge source of income, lower the lower-class taxes, which results in a population increase. this means that new towns will develop. exempting from tax usually helps, especially when the income is lower than 600 a turn. when the new towns develop, a message will pop up, and tell you. click the magnifying galss in the message, and then build factories there.

with 23 regions, in all but the 2 i just conquered, i have all of my towns, most fully developed, and some on the way.



also remember that you do not need more than 2 field armies per theatre. with prussia, you probably have to have 2, but with other factions(like spain or sweden) you dont have anything to worry about, so 1 will do.