It has been said that English is an easy language to learn, but a difficult one to master. Speaking functional English is something many people take for granted, but speaking it well is a rare and powerful ability. Whether English is your native tongue or a second or third language for you, you can improve your fluency and the power of your speech.
Powerful literature is one of the best ways to improve your vocabulary and grammar, as well as giving you a good exposure to many of the idiomatic expressions and cultural references that fluent speech relies upon. Countless expressions in modern English derive from Shakespeare and the Bible.
Just as written literature can be a valuable source for learning grammar and vocabulary, so too can listening to spoken English. Listening to a language helps you develop an ear for what sounds right and what doesn't. Classic movies and famous speeches are good for this. But listening is even more important in conversational English because conversation is dialogue. You need to know what your audience is saying in order to respond appropriately and intelligently. The ability to listen is often more important in conversation than the ability to speak.
Slow Down and Think
Clear thinking is fundamental to clear speaking; you must understand what it is you are trying to say before you say it. This is especially a problem for native speakers of English who might feel frustrated that they are being misunderstood when in fact the ideas themselves are not fully developed. Taking the time to consider carefully your thoughts will often make them much easier to convey.
Whether you're trying to master conversational English as a second language or trying to develop the skills of rhetoric and oratory as a native speaker, you must engage in frequent, intensive practice. Take every opportunity to converse with intelligent and articulate people and don't be afraid to imitate them. Ask them to correct your pronunciation, grammar or vocabulary. It is also worthwhile to practice alone, in front of a mirror. Memorize a speech, or a soliloquy from a play, or a poem, and recite it frequently, varying the inflection and tone of voice to see what effect you can have. In time, the words will flow more naturally.