If you just aren't thinking about keeping your hand up when you throw, go through the motions slowly and EVERY time, snap it right back to where it's supposed to go, then slowly start to speed it up untill it becomes second nature to just throw and come back to where it's supposed to.

If you ever drop your right hand when you are focusing on throwing your left, you can put your right to where it's supposed to be, then stick a towel or a shirt in your armpit and hold it between your inner part of your arm and your body as you throw your left.