oh, and you have to buy the tickets from the ticket office - if you get the direct train from Berlin to Poznan you can buy the tickets in Berlin Lichtenburg station where the train departs from - they speak English there.
I think the Berlin-Warszawa-Express has gone back to Berlin Hauptbahnhof now, but I haven't checked in a while. I know that they were renewing the Oderbrucke, so who knows..
The Kostrzyn connection is absolutely dreadful from Poznan now :(
The buses don't go from Berlin to Poznan (that i know of) but you could get a train to the polish border and then get a bus to Poznan, but to me, it seems like a lot of messing around!
There are some, but finding out information about them is next to impossible! I have absolutely no idea where they might depart from in Berlin...Poznan is easy, everything departs from the PKS station. But Berlin? blaaah.
The best bet is the Berlin-Warszawa-Express, and all details can be found on http//www.bahn.de . You might even be able to buy tickets online there :)