Back Door Bar
A back door bar is a good idea for those entrances that don't have one. Obviously a burglar is going to attack the doors that are away from the main street. This would be the type of bar that fits in a bracket mounted to the door. You have to lift it off the door to open it.
These can only be used by the last one out (who goes out the front entrance) as the Fire Marshall will cite you if it is used while the building is occupied . Then you take it off when you come enter in the morning. Your locksmith can install one and this will make it a LOT tougher for the bad guys to use a pry bar on your door.
This has a "fork" to jam up under your door knob. The bar wedges tight under the knob on one end and down to the floor on the other. It creates tremendous resistance to someone trying to kick the door in. This item is great for travel also. Even if someone has a key they can't get in with it. Adjustable to match up under any knob.
You can find this on Google (click here) prices from $16.00 to $35.00. Or your local hardware store might carry this item.