I don't think you need to. I didn't either time, and both times baby was active enough. Of course drink juice if you want it, but you don't NEED to!
If the baby is being sleepy they may want you to have some sugar there to wake baby up so they can get all the measurements.
I just drank a TON of water (32 ounces in 1 hour to be exact!) and the baby cooperated just fine. After they got the measurements they needed I was finally able to empty my bladder and the baby actually flipped and was facing the other way so that was fun to see. When is your scan?
Jessica--If you get the older u/s tech (with short, white hair) and she tells you that she thinks the baby is a boy, don't believe her!! She told us at 13 weeks that she was almost positive T was a boy, so 7 weeks later, my DH was disappointed to find out that she was definitely a girl. And the other u/s tech said, "oh, she tells people that all the time, I don't know why."