In today’s L.A. Times online, there was a short article entitled A Stretch to Keep Your Hips Flexible. The author recommends a quick and very effective exercise for opening the hips but the pose may be somewhat advanced for the average person just starting out with stretches. Try these modified versions which are easier on the muscles of your back and neck but are proven to be just as effective.
The Exercises
If you have a hard time getting up and down from the floor, then this is the stretch for you as it is done from a chair! 
Sit up straight in an armless chair with your feet flat on the floor, hip’s distance apart. Bring your left ankle to the top of your right thigh just above the knee and hold for a minute or two.
Bring yourself back to starting position for a moment, then switch to the other side.
Note: For a deeper stretch you can bend forward slightly at the waist, making sure to keep your shoulders back and your chest forward. You should not be hunching your back during this stretch.
Now let’s move it to the floor…
Lie flat on the floor perpendicular to a wall. Bend your knees and place your left foot on the wall with your knee bent at a 90º angle. Place your right ankle across your left thigh, just above the knee. Flex your foot in order to stabilize the ankle. Hold for a minute or two and then switch sides.
Note: Moving the trunk of your body closer to the wall will increase this stretch. You can also deepen the stretch by lightly pressing your right knee away from your body.
You can also do this pose with one foot flat on the floor instead of the wall. Lay flat with both knees bent and your feet flat on the ground, hip’s distance apart. Bring the right ankle to the left thigh and hold. To deepen this pose you’re going to do what yogis call “threading the needle.” Pass your right arm through the triangle formed between your legs and clasp your hands behind the left knee. Keep your head and shoulders flat and relaxed on the floor. 
To deepen the stretch, pull your knee towards your chest. Hold for a minute or two, return to starting position and switch sides.
The Benefits
Long periods of sitting each day can tighten the hip flexors, making it difficult to fully extend at the hip. Hip flexors effect the position of the pelvis as well as the state of the back. When these muscles are tight, they pull down on the pelvis which leads to a compression of the back muscles and spine. Regular hip stretches can improve your posture and the positioning of the pelvis thereby reducing back tension and preventing injury.