My own experience with all of this is that I acquired a SI problem back in my senior year of high school. I was preparing for spring track, doing a fair amount of early interval training in a pair of Nike internationalists (does anyone even remember those shoes? that was back in '83) and strained my left posterior tib. Long story short, the podiatrist that I was seeing dabbled in spinal manipulation and beat the hell of of my pelvis with a long-axis iliosacral manipulation. that toasted my training for the next 3 years (frequent bouts of sharp pelvic and thigh pain when trying to train, enough to stop me dead in my tracks) but it also led me to my career choice (PT). I really didn't get consistent relief of symptoms until my first year of PT school. One of my instructors listened to my story, took a quick look at my abysmal hip flexor flexibility and put me on a program of hip flexor stretching which was the start of my being able to train consistently again.
I'd look at the DonTigny stuff, like I noted before. It goes a long way to getting the basics down. I don't agree with a few things here and there (He's against hip flexor stretching, I think that it's essential, particularly for runners but I usually have the runners I treat do so as a lunge style dynamic stretch with a foot on a high step or counter and doing AIS style stretching which promotes posterior ilial rotation on the high leg side while gently stretching the hip flexor of the leg planted on the floor).
I think that SI problems are as much of a motor control issue as they are a ligamentous laxity issue, so training runners to work very regularly (multiple times a day) on making sure to brace their abdomen and tilt their pelvis back to neutral rather than the anteriorly tilted position that they usually favor is important - it's the difference between just doing abdominal /core training and really using that strength to keep your pelvis under your body. I see those faults in runners who can, for instance, hold a prone plank posture for 2 or more minutes easily but who also, when I take side view slow motion video of them outdoors, "cheat" by extending their stride less through their hip than they do through their lumbo-pelvic region. My experience with runners (21+ years as a PT, deep background in manual therapy, now in a practice where I specialize in treating runners) is that it's getting a lot of little things right that gets runners through these problems.
Besides what I talked about above, runners really need strong lumbopelvic control, excellent ability to squat with weight (most of the ones that I see can't perform single leg squats or squats with weight using proper technique to save themselves, nor can they single leg balance well) and a lot of extra work on hip abductors. In regards to running form, the only big thins usually are overstriding issues or low swing leg lift (longer lever through swing that places more load on the hamstrings). I usually encourage runners to make sure to not "sit in the bucket" when running but to make sure to try to lead with their pelvis a bit, which seems to limit the pain.
I think that many runners, surprisingly enough, get into trouble with their sitting posture (of all things) and really have to avoid any slumped sitting alignment because of the stretch it places on the SI ligaments. SItting with a pillow under the thighs to take load off of the ischii/pelvis)seems to help for short periods as long as they don't slouch through the low back. As far as SI belts, I think that they can help - I for one, though, can only wear one for about 3-4 hours before they cause me to ache and use them only for acute symptom control/calming.
I think prolo is an excellent idea if nothing else is getting you there, if you've really put in the hard work to correct the musculoskeletal strength /flexibility deficits. Here's a link for a prolo website if you have questions about it.
http://www.getprolo.com/
Probably more than you wanted to know, but I hope it helps.