Mark,

We use different methods depending on the level of precision and accuracy we need. Sometimes finding points in Google Earth can be good enough, such as road junctions, a recognizable rock, a tree etc. In most applications relative accuracy is more important than absolute accuracy, so you can often get useful data even if your georeferencing is not absolutely spot on. When we need a high level of precision and accuracy we place georeferenced markers made of vinyl window blind panels on the ground. The panels are white and 2" wide. You can form symbols with the panels to identify specific markers. It helps a lot in large fields of crops, where you may not have any recognizable features to differentiate images.