I post a 25 acre irregularly shaped parcel. I use a newer handheld GPS that receives signals from 3 different satellite systems. Most newer units have this capability. I typically a get location accuracy of less than 6 feet but this will vary from location to location. In any case, it's good enough for posting purposes.
Using a GPS requires that you know the coordinates of your property lines. Many older survey maps are recorded using meets and bounds (direction & distance) and it's often difficult to convert for GPS use.
To do this, I use Google Earth with an overlay of my property survey map. There is some minor error but again, it's good enough for posting purposes. I obtain the coordinates of the locations I want to post from Google Earth and save them as waypoints in my GPS. It's then a simple task to navigate to each waypoint and spray or post a sign. This makes it easy to post a property line that isn't straight.