I've been asked so many times to help people with this issue, which I've suffered from myself in the past. Keep in mind that no injury is isolated--you need to address the acute problem first, and then you need to keep hunting around for the root cause.
So herein is everything I've learned about ITBS and what you might be able to do about it:
IT band attaches to TFL (tensor fasciae latae) muscle, AKA ABductor, which helps your leg go away from your body to the side. The band itself is just fascia; so when people talk about stretching it, that's really a misnomer--what you need to do is stretch the TFL [I]muscle[/I]. The fascia may have scar tissue on it though, which is why using a foam roller on the entire fascia can be helpful or just feel good. But my belief is that this provides relief because of an overly tight vastus lateralis, which lies right below the IT band. Tight vastus lateralis usually goes hand-in-hand with weak TFL.
ITBS can be due to a weak TFL, or overly strong ADductors (inner thigh muscles, which act to pull your legs toward midline--there are adductor magnus, longus and brevis) or imbalances between outer quad (vastus lateralis, which also sort of behaves like an ABductor) and inner quad (vastus medialis).
ITBS can also arise due to running on cambered (tilted) roads, which almost all roads are, or running on a track always in the same direction (either leg is susceptible to ITBS depending on whether the TFL is overstretched, weak or tight).
Lastly, underpronators (or supinators, meaning your foot rolls to the outside edge when running--this is common amongst people with high arches) are more prone to ITBS. Sometimes OTC or prescription orthotics plus the proper running shoe (usually cushioned with a bit of stability) are part of future prevention of ITBS, but if you already have it, you have probably incurred TFL issues!
Of course, you need to monitor the mileage on your running shoes, too, as at the onset of ITBS (or anything that hurts while running), the first thing to do is check your mileage.
[URL=http://exrx.net/ExInfo/Inflexibilities.html#anchor13320380]Here[/URL] is another description of ITBS.
Now that you have the complete technical descriptions, what you need to do is seek balance in the strength of your leg muscles and keep them limber. Since running and cycling are primarily unidirectional movements (legs only go forward and backwards in a circular motion), your ABductors may be underdeveloped or weak, which makes you susceptible to ITBS, and your quadriceps and/or ADductors may be too strong in relation to the TFL.
What you can do to strengthen the TFL is spend a few minutes walking sideways on a treadmill or a track once or twice a week, or if you have access to low pulley machine, do [URL=http://exrx.net/Articulations/Hip.html#anchor847041]this[/URL].
If your TFL is overstretched, you need to do a couple of things: stop doing what causes the overstretching. If you run on cambered roads, you need to switch directions to put the affected leg on the HIGH side of the road for a few weeks (trust me, I've tried this and it works). Also, avoid stretching that leg's TFL for a few weeks. Once you resolve the ITBS, then you can begin strengthening and stretching the TFL. The very first hyperlink I provided shows one TFL stretch; [URL=http://www.rice.edu/~jenky/sports/itband.html]here [/URL]is another one, and [URL=http://www.rice.edu/~jenky/sports/itband_str.html]here[/URL] is the one that IMO, works the best.
If your vastus lateralis is tight, almost any quadriceps stretch will work, but you might also have trigger points in the muscle which will need to be resolved. I have found [URL=http://www.amazon.com/gp/product/1572243759/sr=1-1/qid=1138279090/ref=pd... book to be invaluable in maintaining my muscle health. It will teach you about what trigger points are and show you how to identify them and fix them. If you get regular sports massage, it helps if the therapist understands trigger points (the good ones do), and then you can ask them to work on ones you know you already have (for extra therapeutic benefit in addition to your own work) and also identify one you [I]didn't know [/I]you had.
If you have a weak VMO (vastus medialis oblique), which many of us do, go [URL=http://exrx.net/ExInfo/Weaknesses.html#anchor13238820]here[/URL] to read about that and see exercises that can help.
Bottom line is that running will uncover any muscular imbalances, tightness or weakness that you have. If you get a strength assessment and act on it, and then you stretch regularly, odds are that you won't suffer many injuries. But if you do fall victim to ITBS, know that it is usually quite resolvable, as long as you tackle all the potential causative factors.