Keep trespassers off your work site with sturdy construction fencing. If you are working on a residential project, neighbours can be powerful allies. Introduce yourself, give them your phone number and ask them to call you if they see anything suspicious such as individuals on site at odd hours. At the end of the job, give them a small token of appreciation.
If you have a large site, on-site 24-hour security personnel, security alarms or video monitoring can provide additional protection.