Well, it depends a great deal on what you are trying to accomplish. You'd do it in quite different ways on a cadastral map showing land ownership, a thematic map showing a correlation between crop yield and elevation, a topographic map for overland navigation. Scale is going to play a big factor too, as are the constraints of the medium and the information available.

For certain cases where I just want to say "this area is generally use for agriculture" I'm quite pleased with the result I get from chopping the area into evenly sized by irregularly arranged cells (I compute a Poisson disc distribution then find the Voronoi diagram of it) Inset the cells slightly, and then select a random orientation, dash pattern, and spacing for each and draw furrow lines, and then give the lines a bit of random jitter.

Click image for larger version.  Name: farmland.png  Views: 89  Size: 80.0 KB  ID: 60288