Get youself a "Berlin WelcomeCard" by vichatherly
This will save you a lot of time and effort in buying train and bus tickets during your stay, not to mention the discount vouchers to loads of attractions and musuems. You'll be doing well to use up all the vouchers...so check out the sight to make sure you want to go to at least some of them.We bought the 72 hour tickets at 22 euro's each and certainly used up at leat six vouchers and were off and on buses and trains like they were going out of fashion.
"Free travel on all buses and trains of the Berlin-Brandenburg public transport network operating anywhere within the A, B and C fare zones in Berlin for one adult and up to three children (below the age of fourteen) for either 48 hours (16,- ?) or 72 hours (22,- ?)*
The WelcomeCard includes a coupon-booklet which saves you money by receiving discounts on over 120 highlights (sight-seeing, museums, venues, clubs, restaurants, fitness - and wellness facilities and of course for shopping-areas) "
See website for details
Berlin Welcome card