Non-EU Tourists - such as Americans, Canadians, Australians, Israelis etc. - are entitled to a tax refund on goods they buy in the EU and take out of the Union.
There's a processing commission that reduces the refund, but because German sales tax is 19%, it could still be worth your while: The minimal refund is 8% and the maximal refund is 14.5% for goods bought in Germany.
The refund percentage increases as you spend more. You can check out this calculator. There's also a minimal purchase amount of 25 Euro - i.e. you must have purchased for at least 25 Euro - but you'll probably spend more than that anyway...
Here's what you need to do:
1. At the business, ask for the Global Blue tax refund form
2. At the airport, get a stamp on your form from the customs officers - they can ask to see your receipts and goods
3. Get your refund at the Global Blue desk
You can even get your refund upfront before leaving Berlin, in Berlin's most famous department store, Kaufhaus des Westens (that's the one in the picture above). You still need to get the stamp on your form at the airport, but then you can just leave it at the Global Blue mailbox next to the customs desk, which means less hassle at the airport. When you get your refund in advance, you will be asked for your credit card as a security, so if don't complete the procedure at the airport, your card will be charged for more than what you got (there's a penalty).
For more information, have a look at Global Blue's Frequently Asked Questions page.