Here are the things in my routine that i think cured the piriformis:
1) Squats - 2 sets - each set around 10-12 (2nd set to exhaustation)
2) 1 legged presses on the sled - 2 sets on each leg around 10-12 (2nd set to exhaustion). i use heavy weights here and stay with the same weight and try to increase the bend in the knee till you get to a right angle before you add weight. (tougher than it sounds!)
2) adductor and abductor machines (2 sets 12-15)
3) standard quad machine but very light weight, one legged only, with foot turned out. (works the VMO)
4) Figures 8 and 9 in this article posted above
http://www.physsportsmed.com/issues/2005/0505/fredericson.htm
but - stick with the 2 legged version on figure 9 unless it doesn't hurt anything. that's pretty tough on the upper ham. i also avoid the ham curl machine for the same reason.
I only do 2x per week but i'll do figure 8 (plank) at home along with stretching my adductors and hips after running.
I really think this routine has done it. i went down the road of massage, chiro, PT, piriformis stretching, foam roller, tennis ball - you name it, i did it. it wasn't until i got into the gym that it went away and is swear that i had this for like 4 years.
give it a shot and report back. good luck.