Hi Lizzie,
I suffered from this some years ago. It was really sore where the ITB crossed over the outside of my knee, where it had probably caused a painful inflamed bursa. My only treatment was STRETCHING, several times a day, trying to lengthen the ITB to the extent that it relieved the pressure on the outside of the knee. In a couple of weeks it was OK and I could run normally again.
I suppose you've got to look at the cause too. Badly worn shoes can cause it, especially if you're a supinator, or running on roads with a camber. I never did find out what caused mine!
Cheers!
PS. The classic ITB stretch is: Sitting on the floor, bend your injured leg and place the foot on the outer side of your other knee. Gently press your injured knee over the other leg with the opposite elbow. Hold for a count of six, relax, and repeat six times.