Bath regularly and keep a clean appearance. Germany is not known as an unhygienic country, so body odor or an uncouth appearance are likely to be turnoffs for many German girls.
Learn some basic phrases in German. While you don't have to speak it fluently, it will help you to be able to say hello ("hallo" or "tag.") Another useful word is "schön," which means "beautiful." The more you know and understand the better. Get a German phrase book to assist you with the basics.
Recite poetry from German poets Goethe or Heine. Germany is often labeled "the land of poets and thinkers" and both Goethe and Heine were well-respected poets and are well renowned in Germany.
Take her to get German food. Germany is known for its sausage and beer and its citizens are also known to enjoy rye bread and fruit cakes. While there is no guarantee she will appreciate these things, it can be a starting point for what foods she does enjoy and you can then discuss other subjects.