Beginners avoid silence. That’s because they sense that it's in momentary silences that an audience can most easily see through them. So they mask their vulnerability with 'um'.  So, it's understandable but wrong and we all know it.

The goal, of course, is the opposite—to be a presenter in such relaxed control that you can hold deliberate silence in appropriate places. The longer you can hold silence, the more status the audience will award you.

Saying to yourself, “I must not say ‘um’,” is usually a waste of time because you’re investing energy in a negative.

So here’s the point.

Treat a pause like an important word that must be said

That's not just playing with semanitics. Pauses really are important because they give emphasis to what you've just said. Start with a slightly longer than usual silence at the end of a sentence, then progress to silence after commas, then to silence immediately after a word you want to emphasize.

Here's another way to achieve the same effect

Speak in packets

Speak at full normal speed a phrase at a time, but separate the phrases.  An example:  'From now on we'll need to be careful to fill in the forms and to make sure they're handed in in plenty of time.' becomes three packets with two pauses 'From now on... we'll need to be careful to fill in the forms... and to make sure they're handed in in plenty of time.'  Read that out to yourself with and without the pauses and it will be obvious which one carries authority and confidence.

Get used to that and your 'um's will become an endangered species. Good riddance.

Here’s a test for you—you’ll know you’ve nailed it when you can say something really emphatic and then pause, looking from one part of the audience to another during the silence. Make them wait!

If you haven't thought this way before, it may take a bit of practice. Try it out on a friend or colleague.