From the day I closed on my farm it's been a zero tolerance position on my part. Property lines were posted and I made the effort to meet/visit with all my neighbors and exchange contact information. I was also respectfully clear about my expectation they honor boundaries. The first year I called the sheriff twice...since then I've had no issues whatsover.
Whoever said, "Give'em an inch they'll take a mile" is exactly right. Nip it in the bud, have the sheriff pay'em a visit, and my guess is the frequency of trespass declines as word circulates in the neighborhood that it simply won't be tolerated.