I can imagine that it's really hard to learn German grammar (and pronunciation) from native speakers. Most of the regions in German speak a very unique and strong dialect – even when travelling only a few miles.
The only thing that I can suggest, is to let people know that you're not from Germany, but want to learn German. Most of them will at least try to speak standard German.
You might also want to think of specific regions where the dialect is closer to the standard. I would suggest the North of Germany, Baden-Württemberg or Hesse. The dialects are much like the High German (although the pronunciation might not be).
PS: That answer is probably a bit biased. I am from the Saarland (a German Federal State) and its dialect is probably one of the more extreme forms. That's why I am not totally reliable when it comes to German dialects. ;)