There are several relatively painless ways to fix a tic. My favorite is to get someone, a friend, to count the tics over some specified period of time, like a speech, and then charge the offender an agreed-upon sum for each offense. Usually a dollar is enough to get the malefactor’s attention. And you’d be astonished how quickly the tic goes away after you’ve had to pay up a couple of times.
A technique I’ve heard is to have that same friend clap once, loudly, every time you “um”. I’ve seen that work (although I’ve also seen speakers flinch in anticipation, even as they’re trying to stifle the “um”).
But don’t go too far down the aversion therapy road. If a tic’s resisting change, or if you start to see some other symptoms emerge, there may more involved than just a bad habit.
(By the way, Nick’s book Give Your Speech, Change the World is terrific. Hunt it down and read it.)