Luka Slabe, assistant coach on the U.S. women's national team, says it's crucial to use both arms to gain momentum on a hitting approach. Avoid the "chicken arm" and go for a powerful, "big double arm lift" for better elevation.
A simple drill to reinforce this is having players hit balls one at a time and encouraging them to powerfully propel their arms back when taking their approach. If players only half-heartedly use their arms for the approach, have them do a "consequence" such as 5 push-ups or sit-ups.