Use a 3M privacy filter. This obscures the screen for people who are not directly behind you. There are less expensive privacy filters than 3M's.
Choose a booth where you can sit with your back to the wall.
Position yourself so that you’re facing everyone and so your screen isn’t directly reflecting images or text off the location’s windows.
Position yourself so people can’t look through a window directly at your screen.
Boss-Key and Hide are your best friends in situations. As people approach you suddenly, it's Alt-x baby! Once they're gone you can quickly reveal again by clicking the StopWatch icon in the system tray (by the clock in the lower left hand corner of your screen.)