I first had Piriformus Syndrome when pregnant 22 years ago. Have gone through 3 bouts since. I am active and had some "bad" exercises in classes that seem to tip me into it-- too much pressure on pelvis, or injury to sacrum from a fall.
What solved it? Rolling and certain stretches. Much like Loreliks above. Seemed to simple to be true. the technique also worked for some fellow exercise buffs who were finding it difficult to even walk, sit or stand-- we were ready to quit a dance class.
Stretching and some gentle movement in the muscles around the hip warm the area up. The then rolling took out the knots in the piriformus muscle. I first used a soft 6" roller at the Y; I now have a denser knobby one.
For right side: sit with your buttock on the roller. Have the roller at a 45-degree angle to your thigh, lateral side toward the foot and inner (left) side behind you. Keep left knee bent to help you move. Lift your right foot so your leg is straight and about 3" off the ground (may need someone to gently support it). Put your hands on the floor or therapy table behind you for gentle support. Slide slowly forward a few inches, letting the roller roll over the sore piriformus-- you will wind up leaning backward some; then slide back (push with your left foot) so the roller again goes over the sore point and to a spot just below your "sit bone" and you are upright. Repeat 5-6 times as you can tolerate. Twice a day is often enough.
Key seems to be having the roller at the angle to your leg so it hits some pressure points in and around the piriformus. Some of the stretches I learned involve not just back muscles, but loosening the hip flexors and the IT band down the side of my thigh. The time I discovered the roller technique, I was healed in less than a week. Now at the first twinge, I take the time to do the warm-up and rolling and have had no more full-blown incidents. I also remember to walk with my knees and toes forward instead of pointing out to the sides (the movement the piriformus does).

I totally feel for you suffering this! Try the rolling if you can, or get an exercise / Physical therapist to help you try.