This is my favorite hip stretch, known as the figure four glute stretch.
Take lot's of time with this and remember, never hold a position that causes you sharp pain. I use this stretch any time I feel tightness in my low back, hamstrings, or glutes and I hold the deepest part of the stretch for about a minute, spending 5 minutes per side getting into position and doing 3 reps of the stretch. Don't be surprised if you feel a stretch early on in the movement- after a few days of practice you'll be stretching much more deeply.