Good advice from both of the above posters. Another method is to get close enough to a blank target bale that you can be sure that you won't miss. Close your eyes and focus your mind only on keeping your bow arm up. Do this for several hundred arrows. It's ok to let your mind wonder as long as you are keeping the arm up.
When you feel that this is too easy, open your eyes every other shot. Then after a while,keep your eyes open, but be sure that your mind is focused on keeping your bow arm up.
Once you have this mastered, put up a target. Make sure that the target is large enough that you can't miss. Shoot the large target from close up, but gradually move back until you are at the full distance that you want to shoot. Then gradually reduce the size of the target to where it is as small as you want it to be.
If, at any point, you drop your arm, immediately go back to the step just prior to the one where the breakdown occurred.
This is the long hard way to cure any shooting problem that is established in your subconscious. It isn't easy and it is time consuming. However, it works. Once you finish, you will have a new habit ingrained into your subconscious that is at least as hard to break as your old one. The key is to discipline yourself to drop back a step as soon as your shot breaks down and the old habit pops up.
Good luck,
Allen