I always give a slow tendu exercise in first position, facing the barre, before starting pliés. In this exercise, encourage your students to transfer their weight from two feet to one and back again. It is just as important to engage the supporting side from the very beginning of class as it is to warm up the working leg with slow tendus. Make sure to emphasize that they should move their weight over the ball of the supporting foot and back to two feet with each close.
Balancing at the barre is one of the most important things students do. The end of any barre exercise offers an opportunity to work on balancing in a position (first and fifth position, passé, etc.)—don’t waste it! The first one is the most important; it sets up everything else. Take time to be sure the students are properly aligned, using all turnout muscles, and on a high relevé.