im still trying to figure this out myself.
Agrees with the basic principles as stated above
1) Invest in energy. Focus on those double energy resources if you got em.
2) Use your sliders. Gas and Coal are needed but are used for very specific things. you can use your sliders to increase consumption and minimize the stockpiling of those two unless you need them and let your Oil resources fill up.
I would add a few other things to that
3) Increase morale. Production is affected by Morale. Low moral = low output. It is easy to over look. Get your moral in those provinces to 100% or as high as you can.
4) guard against spies. Bah! ive had games that my economy has gone from humming to shambles by enemies focusing on sending agents into my nation and mucking about with my morale and resources. Even causing revolts in newly conquered high resource territories that i had counted on.
5) Be aware of the consumption of your units. High tier units use oil some use a lot of it. If your struggling with oil you may want to postpone building more units that consume or have a high oil upkeep until you have that issue straightend out.
I've only been playing for a couple of months now and still feel like there is stuff here i dont quite understand fully, so perhaps you should take my advise with a grain of salt.
oh... and to answer your question.... I dont think you can "lower the upkeep of population" unless you do lose territories,,,,which is kinda counter productive.