There's a lot of good info on medical websites, just Google it and you'll get some good technical info on the problem. Sorry it took so long to write back, I've been crazy busy lately.
Anyway, here's what I know of Piriformis Syndrome.
-It involves the sciatic nerve becoming trapped & pinched beneath, or as it runs through, the piriformis muscle, which lies beneath the glutes. In some people the sciatic runs through the piriformis muscle, in others beneath it; you're probably more likely to be susceptible to piriformis syndrome if it runs through the muscle.
-It can hurt just in the lower-outer part of your butt (where it pinches) or the entire branch of the sciatic nerve can tingle or go numb. Usually you feel it tingling down your leg but sometimes it imitates "true" sciatica, and you'll feel where it inserts in your lower back on the affected side.
-A common cause is keeping your wallet in your back pocket; sitting on the wallet will irritate the nerve. If that's the case it's an easy fix - don't sit on your wallet! Aside from the wallet thing, if you're putting unusual pressure on the affected side -with a bad chair or if you're not sitting straight - that can be a cause too.
-Running can certainly cause trouble, especially if you have tight hip flexor muscles. Make sure to stretch well to maintain good flexibility, since running strengthens but can also tighten your glutes, hamstrings, etc. If you're having trouble with your sciatic (such as piriformis syndrome) stretch gently.
-Limit your activity to things that don't make the problem worse. If it gets more inflamed after running or stretching, especially if it feels worse the next day, walk for a few weeks or jog slowly, keeping your stride short enough that it's not painful.
-I'd suggest avoiding hills while your sciatic is bothering you. Anything that requires a high knee lift - such as running uphill - really makes it worse from my experience. I actually injured myself in the first place running up a hill while racing a half-marathon - then made it worse because I didn't want to take a break from running afterward.
-As Panic Prone wrote, deep tissue massage can help, as can ultrasound - they may help break up scar tissue. I also know people who sit/roll around on a tennis ball; that helps if the piriformis is spasming.
-If it doesn't resolve in a few weeks, go see a physical therapist who specializes in athletic injuries. They can help with teaching appropriate stretching exercises, giving ultrasound therapy etc.
Just really be careful you don't overdo it and make it worse! Better to take it easy and let it heal with "active rest" than do too much and end up with a chronic injury.
So, my non-expert recommendation is to walk or run very gently until you see some improvement; stretch your hips and the back of your legs - hamstrings, calves, achilles tendons, even plantar fascia (the connective tissue on the bottom of your foot) - regularly; and ice for 20 minutes after exercise (always with a barrier such as a washcloth between skin and ice!) If it doen't improve, see a doctor or exercise specialist P.T.