When we sit too much, our hips get tight. Simultaneously, our glutes get loose. We get what's often referred to as "glute amnesia"...our glutes forget how to get tight or contract. This de-powers us and actually causes the anterior pelvic tilt...not a good thing for athletes...or anyone.
Sean's video contains two very simple exercises that quickly correct this problem. Please do not be fooled by the "home-made" nature of the video. This guy absolutely knows what he is talking about. I've tried out his suggestions before and they almost always work...immediately.
Here is the video: