1. Answer the door. Use the peep hole or window.
2. Have a gun
3. Never open the door unless you are sure about the people outside
4. If it is suspicious, back away to cover the door. Call the cops. Yell that law is on the way.
5. You can get an alarm that sounds a loud siren that you can trigger. Have lights over the door, motion sensitive for the evening.
6. Large animals at your discretion.