As has already been said, rest, stretch and get a massage, the last is very important. I got ITB problems when I suddenly introduced loads of hills into my running in one of my usual fits of overenthusiasm I also insisted on running on it and ignoring it which made it worse though thankfully I saw sense when I realised I was contemplating taking ibuprofen to allow me to run. Its a personal rule of mine to not do exercise if I have to take a painkiller first. Luckily biking was ok and I kept my fitness up.

I did all the stretches I could and took ibuprofen but it was the massage that sorted me out - I am not ashamed to say I nearly cried with the pain , it was possibly the most painful hour of my life (and I paid for the privilege) but it worked. I jetted off on holiday to NYC the day after the massage and spent a weak walking around which wasn't the best plan but once I came back and got off my feet for a bit recovery was quite swift for the ITB issues though I had other problems aside from this.

I stretch my IT Band pretty much everyday now using the yoga pigeon pose.