When Apple demoes desktop applications, they have multiple computers linked to a keyboard-mouse-monitor switch. If the demo fails on one computer, the presenter can quickly switch to a fresh system. IIRC there some videos that show Jobs doing just this.
If you can't afford redundant systems or it would be unelegant to have them around, make sure to make a video or a bunch of screenshots of the planned demo. One of the most fluid "demos" I have ever seen was just a sequence of screenshots. Because the presenter was pointing out things on the screen and clicking on where he would actually have to click to make the next action happen, it somehow felt more like a demo than a slideshow.
The ones I saw, they do not only do that when a demo fails. Almost every single part of a demo runs on a different machine. That minimizes the risk that your demo, by a series of minute deviations from the script, ends up in uncharted waters (this is especially true in alpha or beta software). It introduces a risk of inconsistencies between scene cuts. Those you can minimize by making sure that the 'scenes' at cut points aren't too complex, and by rehearsing, rehearsing, rehearsing.
When Apple demoes desktop applications, they have multiple computers linked to a keyboard-mouse-monitor switch. If the demo fails on one computer, the presenter can quickly switch to a fresh system. IIRC there some videos that show Jobs doing just this.
If you can't afford redundant systems or it would be unelegant to have them around, make sure to make a video or a bunch of screenshots of the planned demo. One of the most fluid "demos" I have ever seen was just a sequence of screenshots. Because the presenter was pointing out things on the screen and clicking on where he would actually have to click to make the next action happen, it somehow felt more like a demo than a slideshow.