The Hip Flexor stretch.
This is the best.
First, let's talk about what the hip flexors are and their function.
They're located at the top of the thigh.
Their function is to flex your hip.
So you use it ALL THE TIME....when you're walking, when you're running....and then when your sitting it gets shorter and shorter and shorter and tighter and tighter and tighter....until one day its so tight it's affecting your posture and rotating those hips in and giving you back pain.
So let's stretch them!
Here's a picture to help you.
So one knee is on the floor the other leg steps forward and you lean (if you can) into the stretch. Another way to really activate the stretching is to squeeze the buns of your back leg. In the case of the woman in the photo, so she could get a deeper stretch, she would squeeze her right buns.
You'll like it!