Vocabulary is always the most important thing, so improving your vocabulary should always be your highest priority. If you already know the words you are going to have it much easier to understand what people are saying, otherwise it can get very frustrating. So for example http://www.flashcardexchange.com/ or www.byki.com are some valuable sites I think in order to increase your vocabulary. And also try to read much and also out loud, this will also help you. But of course expose yourself to the spoken language as much as possible by watching dvds or television or radio is very useful, even if you might not understand what they are talking about at all, by the time it usually gets better.