The first thing to do is to work with them through the "gates" at the barre.  If you dont know what the "Gates" are, ask your instructor.  The gates have to moving your arms through the proper positions in relationship to your feet.  There are some variances between the styles of ballet, but largely once you get moving you will learn them all anyway.

The second thing to do is relax.  The one thing that inhibits your arm movement is stress.  You may not feel stressed but your body hold stress in a few places.  Your jaw, your tounge, your hands, your arms/shoulders, your breath.  If you are holding any one of these your movements will become jagged and less connected.

One thing to work with is making sure you have a good second position port de bras and high 5th.  Once you understand these your first, third and 4th position arms will be much easier to place.  The second position port de bras is really used at the barre, so if your port de bras arent correct you will tense and have a tendency to wobble aimlessly when working your free leg.