Juice before ultrasound?

So I went in last Monday for my anatomy scan, which went great, other than my peanut not cooperating when it came to revealing the gender. I'm going back tomorrow to try again and want to make sure that we don't have the same problem. A few people have told me to drink juice beforehand to make sure that the baby is moving around a lot. So my question is, what time do I drink the juice? My appointment is at 830 so do I drink it before I leave the house? On the way there? While I'm in the waiting room?