You might be demonstrating how to do video editing in Final Cut Pro or showing students how to collaborate on a word processing document using Google Docs. You stop to answer a few questions, look up, and the screen saver has started or the computer has even gone to sleep. Previously, I left myself reminders on my teaching materials to change my Energy Saver settings, but Caffeine (for OS X or Windows)* makes this even easier. One click on the coffee cup (4th from the left in the screen shot below) fills it with coffee and disables the screen saver and sleeping. Click the coffee cup a second time, the coffee drains out, and everything goes back to your normal power management settings. A simple program but surprisingly useful (and free).
*The program has the same name for both Mac and Windows and serves the same purpose but is made by different software companies.