Install a porch light with a motion detector so it turns on whenever someone approaches your porch. If it is a potential criminal, the light should scare him away.
Place a baby gate at the top of the stairs of your porch. At night, it can be difficult to see to manipulate the gate and can keep someone from walking up onto your porch.
Purchase a good watchdog. Any dog that barks easily at strangers is a good choice for a watchdog, even if it is a small dog. Leave your dog in the room closest to the porch so it can alert you if someone is approaching the house. A barking dog will make a potential predator think again before coming up onto your porch.