When conversing in German the toughest thing for me is not understanding what words are being said to me but actually what words are being said. The Berliner accent can be quite hard to understand and takes a lot of practice to get better. In my opinion one of the best exercises in class that we do is listening to the Themen Aktuell CD while reading the  the text in the book.  I find this helps every aspect of my German from vocabulary and grammer to pronunciation and the ever important word recognition skills.