Discarded white electric fence tape can be strung above the top strand of barbed wire, secured to wooden fence posts with fencing staples, or tied with cable ties to star pickets.. If tying lengths of tape together, avoid loose ends if likely to tempt grazing livestock.
The tape offers good visibility, as it is white and shimmies in the wind; it also acts as a physical barrier. It is cheap, quick and easy to put up, especially over long distances. The photos below show Brian Naughton, the originator of the idea, on his property on the Atherton Tablelands.

Photo: Ashleigh Johnson		Photo: Ashleigh Johnson. Landowner Brian Naughton with old electric fence tape as a marker.