Never spent more than an afternoon there but I can tell you that Berlin Transport's website in English is at http://www.bvg.de/index.php/en/Bvg/Start and if you look under Tickets and Fares / Fares Overview you'll see a monthly pass which is extremely good value. For example the zones ABC monthly ticket works out as less than €3 a day - the 1 day pass for the same area is €6.50.