Don't unplug the external drive until it's drive activity light has stopped flashing.
For added safety, use the "Safely Remove Hardware" icon in the notification area (bottom-right) before unplugging.
Don't unplug it at all if you don't need to. I only unplug mine when my PC has shut down for the day, and that's only because the PC hangs at boot time if any external hard drives are connected.