I always ask for a can ("May I have a can of OJ please?")
Works about 90% of the time.
Don't know why but the rest 10% just ignore my request and fill me a cup. This is always the case, they just ignore me.
What follows is a trip to the galley and saying I'm really dehydrated (most people tend to be while up in the air) and usualy end up two cans in my hands.
Pulling the dehydration card works every time. (I drink about 3-4 glasses of water with every meal, 2+ gallons a day)