C'mon Tourist! Dogs are about as close as you can get to having a sentry gun! I say get a whole pack accompanied by landsharks and bear traps
Back on topic though, do you have any motion lights?
I've heard good things about this wireless security unit, it inconspicuously alerts you in the house when someone approaches it--may provide you with a few more seconds to get the drop on a bg if someone comes within 15ft of the transmitting unit. Works inside of cars too