A best practice is to lock you computer screen when you walk away from it.  Use your password to reactivate when you return.  Here is how to lock your screen:

For Windows 7: To immediately lock your computer,simultaneously press the Windows logo key (typically beside the Spacebar) and the l key (the letter L).

For Mac OS X: Activate the screen saver using a hot corner.  Open System Preferences, then Desktop and Screen Saver, activate the Screen Saver tab, and click the Hot Corners button.  Decide which corner of your screen you’d like to use, then click the corresponding pop-up menu and select Start Screen Saver.      Now when it is time to walk away, just fling your mouse into that corner of the screen, and you’ll trigger the screen saver.