As a follow-up to this, my HS English teacher broke me of this by forcing the mantra, "if you are about to say 'um', STOP TALKING'". These works are generally used to fill the speech gap while you are thinking. It seems weird at first to pause, but eventually it becomes natural.
I would posit that people that say "um" more, came from upbringings where they were more likely to be interrupted the moment they left a gap in conversation. That'd be an interesting study.
This is generally not a bad thing, though. From my slight applied linguist background: These fillers are generally used in order to establish that the speaker's turn is not yet done. In a conversational situation, having pauses instead of fillers gives the listeners openings into the conversation to become the speaker.
In presentational speeches, fillers aren't so bad, but long pauses IMO are worse. It's a natural thing to have fillers. If you want to have less, just practice more and be more comfortable with what you're talking about.
My HS English teacher was even more harsh (but effective) - every time someone said um, ah, like, or any other tic/pause word, she'd immediately cut them off. 'STOP.' 'TRY AGAIN.' 'NO.' etc. She was relentless, forced students to stop, calm down, gather their thoughts, and say it directly. Most students were cured in a few days, and the really bad ones a few weeks at most.
