It's always a good idea to have your Mac go to sleep or your screen saver automatically start after 10 or 15 minutes of inactivity, especially if you have the "Require Password after immediately after sleep or screen saver begins" option checked in System Preferences > Security & Privacy. With these options in place, you can walk away from your computer at any time, knowing that within X minutes (depending on your settings), your Mac will automatically lock, preventing others from using it.
While mostly beneficial, having these settings in place can also pose a slight challenge. I find that many times throughout the day, I will be staring at my screen for longer than X minutes reading a document or talking with someone. When this happens, I would actually prefer to not have my Mac go to sleep or my screen saver kick in. I know many would feel the same way. You want the best of both worlds (security and convenience). A common way of dealing with this situation is by occasionally wiggling the mouse to remind the computer that you're still there. I have a much better solution that involves no OCD-like behavior... it's called "Caffeine".
No, I'm not talking about the ingredient commonly found in your favorite soda or coffee. I'm talking about Caffeine the Mac app. This is a very, very simple Mac app that does one thing, and it does that one thing really well. The only reason Caffeine exists is to prevent your Mac from going to sleep or to screen saver mode automatically.
"Why wouldn't I just change my settings in System Preferences to prevent my system from going to sleep or screen saver mode?" That's a great question. In most cases, you want your system to go to sleep or start the screen saver automatically, but not always. For example, if you were reading a long document or showing something to a client that didn't require constant input from your mouse or keyboard. To make this change manually through System Preferences is time-consuming. Caffeine allows you to toggle between the two modes with a single mouse click.
First, we need to install the app (click here). Oh, did I mention this app is free? I love free apps.
After Caffeine has been installed, you will be prompted to set some preferences. The default preferences will auto-run the app for you each time you log in, auto-enable Caffeine (more about this in a second), and show the preference pane each time.
I prefer not to see the preferences prompt each time I log in, so I opted to uncheck that box. Feel free to adjust the settings how you like. When you're ready, click "Close".
Using the app is really straight forward. You may notice a new icon in your menu bar that looks like a cup of coffee.
When the cup is full, Caffeine kicks into action and sends a small piece of data to your system every so often. This information tricks your Mac into thinking there is constant input activity happening (think of it as an automatic "mouse jiggle"). When this happens, your screen saver will never start, and your Mac will never go to sleep.
Clicking on the icon will change the cup from full to empty:
When the cup is empty, it will revert to your normal system settings. When this happens, your screen saver will start after X minutes (which again, is a great security option that all Mac users should use).
Caffeine is one of those apps that should be installed on every Mac. I highly recommend it.