Effective and appropriate communications can be achieved with different audiences by determining the objective of the communication first. Starting a new project at work, for example, should probably be done face to face whenever possible. A conference call can be used if a local meeting is not possible. Following up on a meeting with less formal communications like email and voice mail are probably acceptable. The goal should now be understood by all.
Communications with people that do not know or understand you can be more challenging. Your boss or instructor communications require extra effort on your part to make sure there are no misunderstandings as to your meaning. Because of this, emails should be informative and thoughtful. Coworkers, friends and family probably know you a little better and quick emails are appropriate for them. Avoid email for conflict resolution whenever possible. It is too impersonal and it is very easy to go too far “CCing” someone about a topic that should be handled over the phone or in person. This is personal experience on this one.
Face to face communications is almost always appropriate for any situation. It is by far the best communications option available and should only be avoided when there is an extreme conflict. In this case, you could involve a neutral third party to help negotiate a settlement of issues.
Friends and family communications can include text messages, emails, telephone and face to face. They know you best and even though tone on a text message can be very hard to convey at least you can ask for forgiveness and get it from them. Conflict resolution with this group should be something you can become quite good at since it happens more often than others. A note to your spouse or flowers can work wonders here. I find jewelry to be the most effective form of communication here ;).