I like this article, but especially this bit of advice:
What about your tendency to fill every pause with “Um”? This is telling you to get more familiar with your material. If you know what you’re going to say, you won’t have to pause to remember.
The “um” has always had two meanings to me. One is definitely this. My biggest advice for trainers has always been, look at the material and decide what you want to say, long before you ever get in front of a class. The other place where we see a lot of “um” is in response to questions. Don’t do this. If you need a second to gather your thoughts before articulating an answer to a question, take the second. Silent pauses are not the enemy. Quit stumbling over “um” and “ah” to avoid them. In fact, being comfortable with no one talking is a sign of confidence. It lessens the appearance of nervousness because if you can handle a room full of people looking at you and waiting for you to speak without panicking, you must not be nervous at all!