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Stretch to relax hip flexors: Kneel on one knee and tighten your gluteal muscles (muscles in the buttocks) on the leg you are kneeling on. You will feel a stretch in the front part of your hip. Raise the arm on the same kneeling side and stretch towards the opposite direction. Hold this position for 30 seconds, relax, and repeat again 4 times.

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Exercise to strengthen gluteal muscles: Lie down on your back and bend your knees to about 90-degrees. Squeeze your gluteal muscles together and push your hip upwards till your body forms a straight line from knees to shoulders. Hold this position for 5 seconds, relax, and repeat three times. Do this 10 times daily.

Most Importantly, remember to get up and move around once in a while when sitting and working on a computer.


Sources:
http://www.swolept.com/posts/fixing-anterior-pelvic-tilt-posture-tricks-to-make-your-butt-and-gut-smaller#.Ut5BAPawpGH

http://health.india.com/diseases-conditions/top-5-bad-postures-that-can-affect-your-health/