The tactic i always use is to buy infrantry and artillery. I never try to give up leningrad or Ukraine as you can hold it with infrantry, then buy 3 artilleries per turn to hit back the germans with at least 6 units hitting on a die roll of 1 and 2.
I put weak forces on the front line and my main force behind it to counter attack (for example. put 2 infrantry on the baltic states, eastern poland, and besarabia, then try to put up as many artillery and infrantries behind it to attack them back.)
I always try to lure the axis into splitting their forces (wich is easy regarding the huge chunks of land in Russia) and then attacking them from 2 places at once (for example lure them into Baltic states, then hit them simultaneously from Leningrad and Eastern Poland or Belarussia).
I also try to take Finland and Norway as I find these usually poor defended and gives you, with the NO’s, an additional 11 IPC’s.
But seriously, never forget to buy artillery as you need to push them germans back. Yes, they will take some land from you but putting in a mere defensive line is useless. Use a tactical retreat and you’ll find that they will have a really hard time pushing into Russia.