Here is a link (affiliate) to the bands I recommend: http://amzn.to/1GgBrYh
Get the full protocol for hip mobility from http://bit.ly/2st01Ty
In this video I demonstrate both a standard hip flexor stretch and a banded technique to decompress the spine while mobilizing the muscle. Find the whole post at http://bit.ly/2raduLS
The hip flexors are a group of muscles (iliacus, psoas & rectus femoris) whose primary action is raising the leg. However, one often overlooked action of the psoas is lumbar compression/stabilization. Thus, in order to fully stretch or release the hip flexors, we need to decompress and elongate the lower spine.
I demonstrate the couch stretch, which will help some people. But I spend more time showing a banded technique to open up and release the muscle that may prove more helpful for others.