The night before the exam don't drink any alcohol or too much coffee. Don't smoke weed and don't eat late and get up as early as feels good for you.
Make sure you are well on time at the exam venue. Rather wait 30mins in front of the exam center than having to rush there.
Learn the following standard phrases:
"Das habe ich leider nicht verstanden" = I'm sorry, but I didn't get that.
"Könnten Sie das bitte wiederholen?" = Could you repeat that please?
"Könnten Sie ein bisschen langsamer sprechen?" = Could you speak a bit slower, please?
"Noch einmal bitte." = Once again, please.
"Wie heißt das auf Englisch?" = What's that called in English?
If you are too nervous and don't have any problem with alcohol, simply drink a tiny (!) portion of Schnaps (non-smelly alcohol) and drink it just before entering the examination room, maybe in the toilet or mix it with a bit of orange juice so no one gets suspicious and it'll taste a lot nicer. Alcohol even in small doeses loosens you up. That's a good thing in an oral exam.
More Tips
Do NOT use a dictionary. If you don't understand what you have to do, ask your examiner what it means in English. If they don't help you, simply guess.