I love James. There is so much wise and practical advice in this book. Chapter 3 is certainly no exception, and is a rightfully well-known passage. James is so clear and vivid here that I don't really think I need to say much else beyond what he says. Just a quick thought:
James says that blessing and cursing can't come from the same mouth just like fresh and salt water can't come from the same stream. He paints a picture of the power of the tongue that shows how truly devastating it can be. I know personally that I can think of things that people said to me years ago that I still remember vividly because they were so destructive. The tongue is powerful.
So it seems we have a choice. Will we speak life and build up others, or will we speak death and destroy them?
My encouragement is for you to ask this question of yourself whenever you speak. Realize the power of the tongue and use it for good. Make the choice to not speak death to others or of others. Don't just be neutral either! Actively seek ways to build others up and encourage them!