Trail cameras work very well. They are in the 100 to 150 buck price range. I bought my two cameras from Cabela's a few years back and paid something north of 200 bucks. Like most electronics, they are less expensive and better as time goes by. A camera will hold up to 100 plus pictures, and you can buy bigger storage discs to make them hold more. Like Perry said, you have to willing to press charges. Be sure and conceal your camera, or it will go away with the vandals and thieves. In all of my youthful and fool hardy years, I never thought about vandalizing anything or anyone. The thought of destroying property, especially livestock, is unbelievable to me.
I'm a bit confused by this thread. Are you folks talking about public land. Almost all of our private property in Texas land is fenced. Also, in Texas the trespass laws and the fish and game laws allow for costly fines, like in the 700 to 2500 dollar range. One of my best friends is a game warden, and they really hammer violators. Anyway, the cameras work and good luck to you.
Contact your local sheriff's department and the game and fish agency. They will have suggestions, and in case of an incident they will investigate. Our game wardens are well trained law enforcement officers, and I bet your's are too.