IT band syndrome (ITBS). Probably the most common overuse injury in runners and, to a lesser extent, cyclists. There have been seemingly millions of posts on this particular injury, most of which offer a variety of guidance. I battled a crippling case of ITBS for no less than 6 months. Running meant a reasonable warm-up and 15 minutes of actual aerobic effort before the onset of excruciating pain. If you've encountered this injury, you already know what I'm talking about. If you haven't, there's a very good possibility that you will at some point in your athletic career.
The best description I received is as follows: the IT band is like a rope, composed of many lengthwise fibers, connecting your hip to the front of your knee/upper shin. Whether it's relaxed or tense, the rope is rather taut and varies slightly in length. During exercise, repeated tugging of the rope causes small, localized tearing and inflammation. At the site of these small traumas, the body forms adhesions by building new fibers in various directions. By nature, these adhesions are like knots in the rope - they do not loosen when pulled and they effectively shorten the potential length. This means that your IT band is actually tighter than when you started. From that point onward, any repeated motion will see restricted movement and increased friction at the hip and knee joints - which further accelerates the inflammation and pain associated with IT band syndrome.
I saw 5 different therapists, and tried it all for 6+ months: stretching in ridiculous positions, obscure taping techniques, rolling my outer thigh over various household items, comprehensive strengthening, termination of activity, etc. While I was in a better overall condition, the ITBS never got better - and my running never exceeded 30 minutes. Being desparate for an answer, I finally saw a PT that didn't accept my insurance. Before seeing or treating me, we discussed the nature of ITBS and overuse injuries at great length - information that was of more value that all of my preceding treatments combined.
The treatment consisted of deep, and quite painful, cross-friction massage that was used to break up the adhesions all the way up the IT band - and yes they were about 1-2cm apart all the way up to my hip. He then applied local pressure to the band and flexed it repeatedly in order to elongate the broken up fibers. The treatments were followed by comprehensive stretching and strength training. Some years ago, a similar method was patented by a chiropractor and called ART (active release techique?). The truth is that any responsible PT or massage therapist is completely capable of doing it (without an ART license), but it's the kind of useful treatment that insurance companies frown upon as it's very laborious.
Treatments were bi-weekly (for about 4 sessions), but the maintenance routine is daily. The critical stretch is the hip. Sit with your affected hip against the wall and raise your affected knee up, pulling your foot toward your butt. Straighten your unaffected leg in front of you and place your foot close to the wall. With the palm of your unaffected hand against the wall, push your affected knee away from the wall using your elbow. You'll look like a pretzel, but you should feel a good stretch in your affected hip. I do this on both sides for symmetry.
To strengthen, single leg and turning squats were recommended. Standing on your affected foot and keeping your back straight, squat down and extend your opposite hand forward onto an object 18-24 inches off the floor. Do 15-30 reps, then turn your stance 90 degrees - such that your affected hip is adjacent to the object. This time, as you squat down on your affected leg, turn across your body to place your extended hand on the object. You should really feel this in the hip as you come up - and I've found it to be the most effective strengthening exercise. I also throw in single-hip (stair-step) raises and isolated abductor exercises - much like you'll find all over the web.
It's not a quick solution, nor is it an easy solution, but I am now running up to 16 miles (2 hours) with negligible symptoms. Two months ago, 30 minutes of sub-par running left me in pain for 24-48 hours. I may not win any races this year, but I've got at least one victory under my belt.