Audiences are so savvy today that they often sit and count the number of times a speaker says "um" or "uh" or any other space-filling sound. In a New York Times article, (September 26, 1996) by Sandra Blakeslee, called "Traffic Jams in Brain Network May Result in Verbal Stumbles," linguist Dr. Willem Levelt says that the process of generating thought into speech goes through several layers of networks in the brain. If the process is interrupted at any one of the levels, "...many things can go wrong." And it is especially when our thoughts are not connecting properly that we run into trouble. Dr. Levelt states that "...speech errors can occur in the transition of a thought between the lemma network [which handles syntax] and the lexeme network [which manages spoken sound]." It is at that point that, according to Dr. Levelt, we use "'um' and 'er' to signal that trouble is afoot." In other words, when we have trouble connecting one thought to another, we stumble over our words and shift gears using "um," "ah," and phrases such as "let's see, and "our next point is." Effective transitions help you avoid those stumbling blocks. Look at your outline for a speech and see how many topics and subtopics you have. Each one requires a transition before you plunge your audience into it.
And be sure you don't start your speech with an "uh"—it's a cardinal sin that everyone notices. If you know that you have the "uh" habit, no matter what other speaking faults you have, eliminate the "uhs" first.