Avoiding Panhandlers in Germany?

I'm an American living in Germany. Because of the long commute to my job, I spend waaaay too much time waiting for trains at various train stations in Nordrhein-Westphalia. At least once a day someone approaches me with a hard-luck story about "forgetting" his train fare or something similar (It's the same people over and over for the most part). However, most people standing around me DO NOT get asked.

In the States, simply looking unapproachable (wearing headphones and sunglasses, not smiling, etc.) was enough to avoid these encounters. However, in Germany there must be some unwritten rule I'm violating, because these folks gravitate to me even in a crowd. I'm dressed like the people around me, and without speaking (which I don't), no one should be able to determine I'm American.

What's the secret of looking "unapproachable" in Germany?