You know what they look for? They are looking for some kind of proof that will convince the consul that you will return back to Poland. Best chances are if you have a wife and child, a job because they know you will come back.
I had a sponsor letter written for my bf. It did not help but it wouldn't hurt to have one. They really don't look at it. It used to be that you need an affidavid of support, but they don't require that either anymore.
They are mainly looking for some kind of guarantee that you will return to Poland. The beginning of the year is easier to get a visa than later in the year because they only give out so many and become more picky later. And it also depends on the consul you talk to and you need a little luck too.
Don't go to Scott Renner. He's a consul and doesn't like giving visas.