A good way to stretch the quadriceps muscle group is to lie on your stomach and loop a sheet or strap around the top of your foot. This allows you to pull it over your shoulder, providing a stretch through the muscle.
There are four muscles in the quadriceps group: vastus lateralis, vastus medialis, vastus intermedius, and quadriceps femoris. The first 3 muscles cross the knee joint only, whereas the quadriceps femoris crosses both the hip and knee joints. The prone quad stretch allows you to stretch all 4 muscles groups, since the hip is extended and the knee is flexed.
Hold this stretch 30-60 seconds.