I had a lot of trouble with Piriformis last season. No amount of stretching and PT seemed to work until I discovered what for me is the miracle stretch. Or actually a modification of a stretch I had been doing. Lay on your left side, reach behind you and grasp your right ankle, with your back straight, pull up gently to stretch the thigh. The trick is to push your right hip forward as you do this to engage the sciatic nerve at your hip/back. Hold 15-20 seconds. Repeat on the other side.
My Piriformis symptoms disappeared. I now do this stretch before and after skiing. This is the first season I have not had any pulls or strains requiring PT.