As my teacher once said, "If you can't control your mouth, there's no way you can hope to control your mind.' This is why right speech is so important in day-to-day practice.
Right speech, explained in negative terms, means avoiding four types of harmful speech: lies (words spoken with the intent of misrepresenting the truth); divisive speech (spoken with the intent of creating rifts between people); harsh speech (spoken with the intent of hurting another person's feelings); and idle chatter (spoken with no purposeful intent at all).
Notice the focus on intent: this is where the practice of right speech intersects with the training of the mind. Before you speak, you focus on why you want to speak. This helps get you in touch with all the machinations taking place in the committee of voices running your mind. If you see any unskillful motives lurking behind the committee's decisions, you veto them. As a result, you become more aware of yourself, more honest with yourself, more firm with yourself. You also save yourself from saying things that you'll later regret. In this way you strengthen qualities of mind that will be helpful in meditation, at the same time avoiding any potentially painful memories that would get in the way of being attentive to the present moment when the time comes to meditate.
In positive terms, right speech means speaking in ways that are trustworthy, harmonious, comforting, and worth taking to heart. When you make a practice of these positive forms of right speech, your words become a gift to others. In response, other people will start listening more to what you say, and will be more likely to respond in kind. This gives you a sense of the power of your actions: the way you act in the present moment does shape the world of your experience. You don't need to be a victim of past events.
For many of us, the most difficult part of practicing right speech lies in how we express our sense of humor. Especially here in America, we're used to getting laughs with exaggeration, sarcasm, group stereotypes, and pure silliness — all classic examples of wrong speech. If people get used to these sorts of careless humor, they stop listening carefully to what we say. In this way, we cheapen our own discourse. Actually, there's enough irony in the state of the world that we don't need to exaggerate or be sarcastic. The greatest humorists are the ones who simply make us look directly at the way things are.
Expressing our humor in ways that are truthful, useful, and wise may require thought and effort, but when we master this sort of wit we find that the effort is well spent. We've sharpened our own minds and have improved our verbal environment. In this way, even our jokes become part of our practice: an opportunity to develop positive qualities of mind and to offer something of intelligent value to the people around us.
So pay close attention to what you say — and to why you say it. When you do, you'll discover that an open mouth doesn't have to be a mistake.