hello.
I am not an expert playing with Poland or Lithuania, I usually play with European or Cossack factions (moreover, recently, I was not playing much)
... but I played a mini-campaign with Poland, and advise you from that experience of mine, your effort center to take care in cities of the west ( Lublin, Varshava, Wroclaw, Gdansk, Elblag, Chelmno... all these), the basin of the Vistula (I think that is the Vistula, for her towns that surround it). In the cities of the East, well, give them no respite fight, but neither will obsessions with them, if you lose any, well take it easy, can retrieve later ... The flourishing cities of the west more quickly (in economia, more than anything) and less trouble... To me, each general (character named) appeared to me, sent him to a city in the west.
I hope you will help
EDIT:
... hummm. Try to create two great armys, one in the west and the other in the Prusia region... remember, how in all TW games, the citys with sea ports generate more money... maybe you must think in a war with Prusia, her cities are very much richest...
greetings