Add this exercise to your warm up once or twice a week.
This is a dynamic stretch which will move your hip flexors through their forward range.
Imagine a toy soldier marching with extremely exaggerated steps.
Stand tall, looking forward.
Stretch your right hand to the sky.
Now lift your left leg with your toes pointing to the sky. At the same time bring your right hand down to meet your foot
Now stretch your left hand to the sky. Bring your right leg up and left hand down.
Continue this movement to do 10 paces on each side.