I always maintain a strategic reserve in my core provinces, usually consisting of a combination of my legacy infantry, as well as non-oil-burning defensive units such as anti-tank and anti-aircraft. Also, if you have the upfront production resources available, commandos also make a good defensive unit because they consume the same amount of food as militia do, 55% of what regular infantry consume, they are stronger under most circumstances, and they have more hit points (25 vs. 15). Consider combining 2 commando battalions in a city (+25% strength points) or mountain province (+100% strength points), with 2 anti-tank regiments and 2 anti-aircraft regiments. If you have some quick response fighter wings to provide them with patrolling air cover, that forms one heck of a defensive hedgehog, and a nasty little surprise for any invader until you can crank up your industrial production.
I also invest heavily in my industrial production capacity, with every core resource province fully built-out with L3 infrastructure, L5 industrial complex and L2 air bases. I may not always have the largest army on the board at any given time (in order to avoid unnecessary resource consumption), but I almost always have the largest economy and the largest production capacity by Day 14. Which means I can often double the size of my fighter and tactical air wings in a very short period of time (36 hours), and nearly double my armored ground forces (48 hours).
Most if not all of my core resource provinces are usually fortified to maintain them at 100% morale when I am fighting two wars simultaneously, so my reserve defenders also have an additional measure of protection working in their favor.
And wherever my Angry Birds tactical air wing may have wandered in pursuit of expanding my empire, I always maintain a string of L2 air bases that will enable all or a significant portion of my tactical bombers to return to my core provinces within a few hours to defend the homeland.
Every long-time player has found strategies that work for him. Maxing my resource production and industrial production capacity, not maxing my army size until I need it, and rapidly expanding when I'm threatened, is the overall strategy that has worked best for me.