ORIGINAL: Drakken
Yes, I figure that it is a matter of experience. Hence why I ask.
I think I'm not nearly experienced enough to give any solid advice, thus my enigmatic answer
I just mean that there is no single advice that would cover all defenses, mostly you have to improvise with what you have especially on the Soviets in the summer '41.
One solid advice for Moscow defense is that no matter the cost, I would prepare at least some kind of defense ring around Moscow somewhere in 20-50 mile radius (Nara-Mozhaysk-Lama for example). Any defense set up at Vyazma-Rzhev is likely to collapse, so you will eventually need forts near Moscow.
For the defense line in July '41, I'd just try to make sure that minimum amount of units would get surrounded or zoc-locked. Stacking units high is a risk since the germans can just go around and at minimum zoc-lock the strong stack. Making a carpet defense has a risk if there's lot of panzers around, with the amount of MPs they have at use, they can just use hasty attacks to create a large pocket. Reserve activations are really crucial to bog down breakthroughs as someone mentioned earlier!
So basically it's juggling between options based on what knowledge you have (your opponents style, amount of troops present on both sides, terrain, weather on next turn, your own plan where the future defense line will be, etc.).
I really wish I had the time to make comprehensive tests on defending with different formations under different conditions... but I don't, so I'll just stick to vague answers
