I was staying in Warsaw and took the train and booked in with this tour group http://www.warsawtraveltours.com/en/adventure/wawa-tours/tours-from-wawa/Auschwitz-tour/30.html. They were really excellent, we haggled a good price for three of us.
Things I wish I had known before going:
* Auschwitz is quite small.
* Birkenau is what you're expecting in terms of size.
*Auschwitz: the 'museum' is in remodeled buildings...
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