I recently did a post on how to loosen up tight hip flexors. It focuses specifically on how to get mobile through the front of your hips, however a recent comment that I have had reminds me that everyone has different problems.
I also run a site on Volleyball Conditioning, and have recently done a post on how to get more range through your hips, as it is actually quite important for many skills in Volleyball. The post has a lot of exercise examples, pictures and videos on how to general loosen up what is a pretty tight area in most people. Check it out if you have problems associated with tight hips or simply want to get more range through your hips in general.