In a lot of states purple is the same as no trespassing signs.Ya but paint in no way means "no trespassing". Put the signs in a tree ~10 feet off the ground...where they are still in clear view but can't be ripped down. That coupled with wireless cameras works well for us.
It would be interesting to know the length of the OP's line. In conjunction with some of the techniques mentioned above, if the corners can be posted on a photo you can use natural boundaries and/or roads to adjust along the length of the line. We sometimes will have a surveyor put control points with his GPS every 1/4 mile or so along a line... even if they are adjusted later we can use the offsets to run a straight line.