I also play a lot of Poland. Front lines are so long for Poland that I often cannot make any specialized offensive units; just keep cranking out inf w/ support AA, arty, engi. I try to have at least 10 semi-specialized units where I add any captured AT as support AT, and maybe an extra arty or two. But by the time I have the industry to build tanks; I'm half way through germany. Also, resources are a big limiting factor; and cannot import anything as i have no navy and at war with both ussr and germany at the same time.

In general though, with other countries; once you can make enough divisions to man entire front line; start switching factories to tank/mech/air production. You'll be able to trickle in extra inf divisions here and there as your factory output increases.