Speak only in German, all the time. It might be hard and you'll be limited by what you can say, but you'll get creative with learning to express yourself and will really cement your grammar and vocabulary. Speaking all the time in a language is crucial to learning it, so no matter how painful or awkward you might think it will be, just spend the majority of your time speaking in German, ok? It will do your learning wonders!
Also, if you have time, check out as many things in Berlin as you can! Museums, sights, culture, food, etc. Don't waste your time on things that you could find at home--immerse yourself in the local area!