To address the “what stretches?” question; there are 3 that i commonly suggest. We will use the Right IT band for each example. Take care to relax into these stretches; remaining tense will diminish the effect.
1) Lay supine (on your back) and bend your knees to about 90 degrees with your feet still on the floor. Place your Right ankle on your Left thigh and let all your muscles relax. Lift your Left leg and grab your Left shin with both hands. Try to relax all muscles except for the slight pulling pressure from your arms. You’ll feel a lot of stretch in the glute as well.
2) Sit upright on the floor with your legs straight in front of you. (Should look like an “L”) cross your Right foot over to the outside (lateral) your Left thigh and attempt to put your Right foot flat on the floor. (Do the best you can) Put your Left elbow on your Right lateral knee and your Right hand just behind you for balance. Now, use your Left arm to apply pressure while twisting your body to look over your Right shoulder.
3) Pigeon pose is a yoga posture that happens to target the ITB very well. If you are unfamiliar with yoga, I suggest getting someone experienced to help you into the correct position.
I would suggest that you start with an overhead squat assessment and see what other muscles may be compensating also as it is rare to have just 1 muscle/tendon cause all the issue.
However, for the IT band I use foam rolling. Look for a video by Eric Beard on youtube about it. He is amazing – teaches for NASM, years of hands on experience, etc. You may also want to “look up and look down” at muscles at the hip and in the lower leg which may be causing the IT Band to get “tight” as it compensates for other muscles not doing their job.