I will say this in the spirit of somebody who started competitive public speaking as therapy to overcome a speech disorder which would have been virtually disabling professionally: you can hack your way through hesitation noises. A lot of the HN comments suggesting practical ways to do so would be effective. ("Speak slower" and "Use the air gap for a dramatic pause" are my two favorites.) Practice plus directed effort will very quickly make this not a problem for you. It is totally not a given that you will revert to habit when not paying attention, when speaking off the cuff, or when saying interesting and important things. You just need to get into a new, successful habit, just like you long ago cultivated a habit of e.g. not spelling words wrongly.

(n.b. I have issues with hesitation noises myself occasionally. When I have the opportunity I watch / listen to tapes of myself, count influencies, and avoid things that cause that number to spike, because improving on this is a priority for me. For example, the worst I ever did last year -- TwilioConf -- had 1/20th the incidence of my typical performance in middle school, and on good days you wouldn't be able to tell I'd actually struggled with this.)

[P.S. This is going to sound a little fluffy but it is absolutely true: one of the first steps is to stop saying "I am not a good public speaker" and start saying "Some of the speeches I have delivered have had a lot of umms in them", because that identifies a specific issue which can be fixed by an identifiable behavioral change, rather than solidifying an identity around features of past speeches you may have made.]

I believe I've heard of linguistic studies which demonstrate that the use of "filler" words like um is, in part, a form of "code switching". For example, when teenagers talk to their peers, or to people they don't particularly respect, their speech will tend to be peppered with ums and you knows and likes [1], as in: "I'd, um, you know, prefer to watch something, like, interesting.". But when talking to respected elders, or working as waitstaff in a restaurant, the same people suddenly prove capable of uttering perfectly fine sentences in formal English. The "reversion" to the broken-up speech pattern is not necessarily because the speakers are thinking too hard, or drinking too hard, and it's certainly not because they don't know how to speak: It's often a subconscious but deliberate technique for setting an informal mood.

Some languages actually have a syntactically-distinct informal mode; English has the word um.

Why do I tell this random anecdote? The usual reason: I love to ramble. But also because my point is: Stop worrying about the "ums". They're fine. Your listeners were probably already interpreting them as a sign that you've become really engaged in the conversation.

(Now, of course, the above comment will be added to the YC FAQ, and teams will be competing to see how quickly they can get PG to utter the first um of the entrance interview. ;)

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[1] At least, this is what informal teen speech sounded like years ago. These days, it may be different. It usually is.

My father grew up in a very rural, very uneducated part of the U.S. and had a deep "southern" accent (though "country" or "rural" is probably more apt). When he was 16 or so he ran away from home and landed in Chicago where his accent immediately marked him as a country bumpkin. This made him tremendously shy in the city and he spent most of his time in isolation.

He spent years retraining his accent, reading Webster's, practicing day in and day out. To this day he can't even successfully fake the accent of his youth. My best guess as to what he sounded like comes from his brothers, all of whom have a rich rural accent to this day.

As part of his effort to get over his shyness and his accent, he joined several public speaking clubs. While they helped with his accent and his shyness, they also taught him a great deal about hesitation noises and he's passed on two things to me (which of course I didn't truly appreciate until years later when I had to get out and start talking in front of groups).

1) Hesitation noises are usually just a way for our brain to catch up with our mouth, interestingly and helpfully, it's also a way for an audience's brain to catch up with their ears.

2) It's funny becoming aware of them, it's like suddenly becoming aware of breathing, it's very automatic, and when you start to think about it, the first thing you do is take a deep breath. The same with ums and ahhs, once you are aware of them, the first thing you should do is take a deep breath instead of saying them.

The audience will be glad for the pause, they'll have time to absorb the complex thought that you are in the process of putting together anyways. And you'll have time to put it together.

If the deep breaths aren't your thing he also recommended I turn my "ums" into "mmms" and then the next step is to turn the voiced "mmms" into a silent outbreath through my nose with my lips in the "mmm" position. It doesn't make a distracting noise and the effect is a thoughtful pause.

Just trim the length of the ums and nobody notices them anymore. Fund a startup that can do um-filtering (speaking through a microphone) on the fly, problem solved. :)

You are in great company, just watched a couple of videos of Elon Musk and Donald Knuth. They are also expert um'ers but everyone keeps listening because the message they are broadcasting is interesting.