I would also recommend getting a monitor that let's you know when someone has driven or walked across your driveway. This gives you a head's up BEFORE they get to your door.
The Home Security Superstore - HomeSafe® Door & Driveway Alarm - DW-ALARM - Home Security Solutions
I bought that same one for around $17, just for a spare. Loaned it to my neighbor, single mom with medical problems. She's still got it, but I figure I can always get another as cheap as they are. They do work pretty well.