One of the biggest faults of American speakers is that we give everything equal emphasis. Read the following paragraph from a story by W. Cabell Greet, associate professor of English, Barnard College (and if possible, record yourself doing it).
Once there was a young rat named Arthur, who could never make up his mind. Whenever his friends asked him if he would like to go out with them, he would only answer, "I don't know." He wouldn't say "yes" or "no" either. He would always shirk making a choice.
What is the most important point in this paragraph? It's that Arthur could never make up his mind; the places in the paragraph that make that point should be emphasized. The others can be read at a fairly quick pace.
Vocal variety is one of the most powerful weapons in a speaker's arsenal; good speakers use it to great effect. If your words were written, you would rely on punctuation to move your thoughts along and to link your ideas. In speech, all you have to make your points and to get people to understand is your voice; it is the only sort of punctuation the speaker has.
An effective speaker stresses points by limiting the main ones and by shading all the rest through vocal technique. The key words you emphasize, the pauses you insert, your shifting pitch, rhythm, loudness, tone, and rate of speech all affect how your audience interprets your words. Learn to build; even lists can gain drama if you let your voice add it. For example, "I came, I saw, I conquered" could be said with equal emphasis on the three verbs, and it would seem rather bland. But "I came, I saw, (pause) I conquered" builds to a finale and catches the audience's attention.
You should always "lean" on the new idea in your sentence. For example, if you've been mentioning sales success and are introducing the concept of life success, emphasize the word life vocally, because it represents a new idea, and de-emphasize success, because you've said it already. Nine times out of 10 an action word (often a verb) represents the new idea; don't leave it entirely up to the audience to grasp the new idea just because it's new; use your voice to nudge the audience along.
Another way to highlight an important point is to stop talking just before you introduce the point. The pause is one of the most effective attention-getters around. People get so used to hearing a speaker talk that when there is silence they sit up and take notice.