Lots of good suggestions, but I particularly like the one from "Adirondack Pond" regarding the use of inexpensive motion detector alarms. It would be best if you could place a couple of them somewhere that would be hard to quickly access. No one in their right mind would stick around with an alarm going off, especially if you had no trespassing signs posted that also warned of video surveillance. Anyone who would not be deterred by this is either someone you know, or is someone so crazy that you'd probably not want to confront anyhow.
