Move a small trailer to the property and pay someone to stay on the site
 Fence and post the area-you can use a guard dog to help secure the area
 Change the locks on your site often
 Install security cameras and notification that the site monitored
 Gas caps and oil caps should be locked-this will prevent against gas theft but more importantly, this will prevent against sabotage

Establishing a relationship with the local scrap metal companies might help you to catch criminals if they try to sell your metal. Aggressively prosecuting thieves after they are caught will send a message to other, would be, thieves.

Of course, there is also petty theft that occurs among work crews. To avoid problems of who belongs to what you can have some security rules on the site

 All tools are clearly marked-there are plenty of ways to mark equipment. You can offer an electric engraver for people who have not yet marked their tools.
 Unused equipment is placed in a locked tool shed at all times
 All vehicles on the property will be locked at all times
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At certain times, there will be heavy equipment on the premises. To protect the large equipment from theft you can use hidden switches that disable the vehicles. This will prevent "drive offs" by enterprising criminals. It is also a good idea to not leave equipment in remote areas. If you own the equipment, try to have your heavy machinery re-keyed after every job. It is not unheard of for a laborer to make a copy of a key during construction and to return and steal your machinery long after the construction is complete.

One of the most important things that you can do to secure your job site is to hire good people. Hopefully you will find an honest and skilled general contractor. The contractor will be in charge of hiring the subs. If he has been doing construction in the area for a long time he will have established relationships with plenty of subcontractors and will know who is trustworthy and who isn't.
If you have any questions at all about the persons on your construction site, you have the right to conduct a criminal background check. The price of running background checks is small in comparison to the price of replacing expensive equipment.
Finally, don't tempt fate. Especially when it comes to vehicles and appliances. Work trucks that sit on the site with the keys in the ignition are begging to be stolen. Don't be surprised if this happens. The same is true for appliances. If your appliances arrive before it is time to install them, store them somewhere else. A brand new dishwasher sitting around on a job site is an obvious target.