Originally, I considered using PVC union fittings, but they are quite expensive and bulky. I finally settled on an inexpensive solution using expendable parts that can be unscrewed from the valve body and risers.
The first photo in this series (above) shows the original backyard valve installation, with some earth dug away to expose the pipes. The valve on the left has been replaced at least once already, requiring the riser to be cut and material to be spliced on to bring the valve back up to its original height.
As you can see in the photo, I'm using a scrap piece of 1x2 lumber as a stop so that I get absolutely uniform lengths of PVC pipe. You can use the same trick with a miter box, using a C-clamp to hold the stop in place.