One of the most important and forgotten aspects of being a successful, healthy runner is stretching/flexibility training. Studies have shown that a proper warm-up prior to a run followed by specific stretches for running will help reduce injury and increase flexibility. A few of the most important areas to stretch are:

The Illias psoas (hip flexor)
Hamstrings (back of thigh)
Quadriceps (front of thigh)
Gastrocnemius/soleus (calves)
Lower back & Abs
Gluteus maximus/minimus/medius