Very painful to watch. I hope your clerk will get counseling if she needs it. As a 20 year policeman here are a few tips that can help prevent this in the future.
Get a friend of a PD to give you an old hat. Then in plain view place the hat behind the register area and sit a cup of coffee (empty cup) to give the appearance he's on break. Make sure it's in plain view and you will be surprised at the looks and comments you will get.
Then on the front door place a sign "videotape on premises". Then you can go out and buy a few dummy camera's, when they read that sign they "criminals" always look up and if they see a camera they don't come back. Over 90% of the robberies people have visited the business (like yours) and they look for what security you have. Be proactive and always be on the lookout. I personally would have at least 3 cameras live in the store for various reasons.