Junipers are pruned in a different way decidous species do. If you were to use a pair of shears to cut some foliage, the ends of the needles would get brown and that isn't nice to see. By pinching( pinching means to use your thumb and index finger and pull the foliage you want to remove) you avoid the browning of the foliage. You should remove foliage in steps depending on how old the tree is. For a young tree i would remove more than 20% of the foliage at the time, but for an old tree you can remove up to 40% of the foliage at a time.The same rule applies if the tree is healthy or not. Here, take a read at these:
http://www.bonsai4me.com/Basics/Basics_Pruning3.html
http://www.evergreengardenworks.com/pruning.htm
http://www.bonsaiprimer.com/pruning...er/conifer.html