Identify the issue. Ways to improve your speech a great many, but there is no need to waste time on something that you have developed. It will be easier to achieve the goal, if you break it into smaller tasks and solve them systematically. Moreover, making a small list you can more clearly feel your progress (or lack thereof).
Develop your diction. To solve the problem of bad pronunciation try to say with clenched teeth a cork from the wine. However, using this method, beware of the plastic "counterfeiting" under the tree, they can damage the teeth. An alternative and complementary tube can be 3-4 walnuts in your mouth. This exercise is similar, but still different from the first because you should try both.
Take some vocal lessons. There is such a thing as "voice power", without which any performance on stage will be a failure. To give your speech the credibility and volume, you need to "withdraw" the voice outside. Try to not speak the vocal cords and chest, how would using the entire body mass, then the sound will be more powerful (try to copy Levitan). Singers use the same technique to "put" a beautiful and rich voice.
Work on the vocabulary and sentence structure. There are many examples of not only how to speak but what to say. After all, your speech will be considered correct only in the case if you reach the said desired results. Great tutorial, in this sense becomes the book of Allan Pisa "Speak correctly..." and the film "Thank you for smoking". The rest of you can only give basic advice: read more.
Think, what conditions you speak. When speaking from the rostrum, for example, has its own peculiarities that must be considered in order not to lull the viewer. The main point is that it is impossible "to read" - your proposal should not contain complicated turns, fine speech or abundance of interjections. In such conditions, "articulate" means to speak freely, almost like in everyday life - but at the same time "delivered" a confident voice and diction. From the speech of the speaker is a sight to behold, but the main thing that you will learn to enjoy being on stage.