for me, I can destill it down to 2 main points.
1- Don’t lose big battles
2- build your armys stayingpower.
1- Don’t lose big battles
Losing battles is how you lose your advantage. If Moscow is doomed to fall, and you will lose 250 IPC in units, while the german will lose 180 IPC in units, then you are better off retreating from moscow, if you stand behind moscow, then you force the german to garison against your army, if the german army wanna move towards the middle east, then forcing him to keep 200 IPC worth of units just to garison moscow is worth it. if he moves to much away, you can come back and retake moscow.
Similarly, you should not move your fleet to a place where it can be attacked and killed, if US lose their entire fleet for 80% of the japanese fleet, then japan has a free reign for a few turns and it could easily lose you the game, similarly around europe.
2- increase your armys staying power. when advancing the question usually isn’t if you can take the terretory you want to attack with your main army, but whether you are able to survive a counterattack. When you are advancing you are advancing away from your reinforcements and towards your opponents reinforcements. Imagine that Army A has 55 units and produces 10 uniter / turn. in round 8. If army A retreats It will have 75 units in round 9, but if it attacks it will have 55 units in round 9, every advance “costs” a turn of reinforcements (if playing with inf/art). So lets say army B wants to force army A to retreat and for that it would need 65 units, then to follow in after it would need 75 units. This is why volume is so vital in AA, and why mech/inf is the two most important unittypes you will ever produce.
other issues;
-Tanks are specialist units against a good player you will very rarely get an advantage from being able to blitz with 10 tanks + 10 mechs, and this advantage would come of the cost of having 20 inf + 10 art, which is good when you need your main stack to advance. I think the only power that could benefit from a tank/mech stack is japan, when they are trying to chase down the siberians, but then such a stack would only force the russians to leave 1 inf behind for each time it moves to counteract that, and you would need do dislocate your airforce to force him to do it, which probably means that it costs more than it is worth. What you need tanks for is to be able to blitz empty terrs. Usually 2-4 tanks would be enough for most powers. if you have canopeners, like US can canopen for UK, or italy for germany, then you only need mechs.
- it is often better to have an army on a fleet than to go ashore. If US have 15 transports with loads standing of gibraltar, then you force the axis to garison with 30+ units in W germany and S italy, and having units for counterattacks in france and N italy. Once you go ashore, he can move some of those units (like his planes) and your threat is gone.