ITBS is IT Band Syndrome, and it's very common in runners. The pain is usually on the outside of your knee. It's caused either by overuse or biomechanical problems (ie. bad shoes, etc).
This is what worked for me. I'm not a medical professional, you should consult one before doing anything, as well as to troubleshoot the cause of your problems!
Don't Run in Pain
Don't try to run through it. If it hurts, stop. Immediately. Every little bit you run with the IT rubbing the knee does damage, and it can take weeks and weeks to get better. In my case, I ran in a duathlon while it was a problem, it hurt for the last 2.5k run and that set me back about 6-8 more weeks. Bad idea.
I had excellent luck with cycling, no pain even when my ITBS was at its worst. There's also swimming, or running in the pool (less impact). I took a complete break from running... other people are able to keep running short distances pain-free, but the key is to stop before the pain starts.
Ice and Ibuprofen
... for when it hurts.
I have had the most luck with this one... you really need to feel it on the outside of your leg, if you don't then move your torso forward (from the waist up) once you're bent over.
Even when I'm not having ITBS problems I do this stretch for prevention.
Buy a pool noodle, lie on it, and roll with all your weight resting on the noodle. Roll from the hip to the knee, slowly, and if you get to a point where it hurts stop and take the pain. Yes, this hurts, especially if your case is acute, but it helps get that IT band loose. I still do this when it starts feeling tight on me, seems to help a lot.
Slow Return to Running
For me, I had to start almost from scratch... 2K run-walks. It was ugly, but whenever I tried to do more it would flare back up. I maintained my fitness through cycling, and now I'm back to my normal running and distances.