Open, constructive communication is important to re-establishing and maintaining a healthy family dynamic, during and after the addicted family member's treatment.
Healthy, open communication includes clear and specific statements. Be problem oriented, not person oriented (don't blame the addict, blame the disease). Be respectful of how others feel and be honest about your own feelings. When the lines of communication fail, consider family therapy -- at the very least, the therapist can help you begin to communicate in a healthy way and can mediate difficult conversations.
Constructive communication isn't all about what you say and how you say it, though. Remember to be a good listener.