I can't remember what it's called, but stretching a muscle group will over time relieve long standing tension in the opposing one. So- Hams/quads, glutes/TFL and psoas, adductors/abductors. I was told it your body's way of balancing up, it tenses the opposite rather than releasing the tensed one...if that makes sense.
I always pull into the last little bit of a double leg hamstring stretch by tensing my quads, this automatically loosens up the hamstrings during the stretch.