the most important recommendation I give to the students of German is: Learn the grammar properly and thoroughly from the very beginning.
It really pays. Catching up later is extremely annoying and painful, in my opinion much more than in other languages because of the complexity of the German grammar. I actually know many foreigners who easily communicate in German and are understood despite the huge amount of mistakes they make. None of them, however, was and is able to do away with the twisted grammar because they were not careful enough in their first 20 lessons (things like "mit + Dativ", "alle guten Freunde", "einige gute Freunde" etc.).