The ideal way to prune a spreading juniper is to take off any deadwood and then stop. What makes a juniper attractive are those long, feathery branches. The reality is, however, that many of these plants have been placed in locations that they can’t be allowed to spread forever. Unfortunately, once they start to grow into driveways and over sidewalks, most people start chopping them off. The term “hacking back” comes to mind, and with a juniper it doesn’t usually end well.
Let me suggest an alternative approach. Early in a spreading juniper’s life, before it grows into that driveway or sidewalk, lift the top layer of branches and cut the older, lower stems back into the plant. The top growth will cover where you’ve cut, maintaining the natural look of the plant.
Once the shrub is even older, however, this stops being effective and those lower, chopped limbs start to become visible. This is the time to start thinking of a spreading juniper as a very large bonsai tree. Begin to arborize such plants by cutting those branches back to the main stem, exposing the trunk. This can create interesting, sculptural forms that maintain their feathery nature and stay away from passageways.