There are times when it is desirable to prevent sleep mode, screen saver activation, etc. For example, during Software Update activity. However, it is inconvenient to temporarily change settings in System Preferences temporarily. There are other hardware and software solutions to keep your machine awake, such as Mouse Jiggler.
I offer a simple solution for those with an optical mouse: a watch or clock with a sweep second hand. Simply place the mouse on the face of the watch in such a way that the eye of the mouse can see the second hand pass beneath it. Every 60 seconds (like clockwork -- oof!), the cursor will move a few pixels, refreshing the display.
Mac OS X Hints