http://www.central-alarms.com/alarms...portable_.html
Attach the pull cord to the sign leg, anchor the other end into the lawn with a tent spike.
Watch the chaos ensue when the alarm goes off. You should be getting to the window juuuust in time to see him start looking for that skin he just jumped the hell out of.
Option #2 is to get a laser pointer and sit in your window with it. Laze the forehead of whoever crosses the "Line of Death" in your own personal Gulf of Sidra.