I posted in the "sciatica" thread that I have previously had SPD in a baad way with my two boys. It got so bad with DS2 that, at the end, I couldn't grocery shop without those motorized carts. And that was in spite of regular, high-quality chiropractic care.
I have a friend who is a PT and she taught me some self-care exercises/practices. Her specialty is pelvic issues! So I totally grilled her on how to handle my pain that was starting so early. She said chiro *can* help, but it often doesn't. She showed me a self-adjustment exercise to do up to 3x/day: Sit comfortably with your feet flat on the floor. Place your R hand on your L knee, and lean over so your R elbow is braced against your R knee (so your forearm is spanning the knees, basically). Do the opposite with your L hand/elbow so your arms are sort of criss-crossed between your legs, if that makes sense. Squeeze with your inner thighs for about 5 seconds or so at 80% strength. You can do up to 3 reps at a time, but don't go crazy and act like it's a workout or anything. The motion should realign your pubic bones and strengthen the muscles that keep them there.
She also recommended some kind of special belt (it was a different name from anything I'd been looking at) that fit tightly around the pelvis/hips, but to only use it once the bones have been aligned, or it makes the misalignment worse over time.
She said some basic things to do/avoid are: sleeping with a thick pillow between your legs, and if you are a side sleeper who flops your top leg over, be sure to never let the knee drop below mid-pelvis (so again, just move the thick pillow to rest your leg on it instead of letting it hang); don't cross your legs while sitting if you can help it, either at the knee or ankle-over knee.
..I haven't done the belt...haven't needed it, but do the pillow without fail and the exercises almost every day. I have barely had a hint of SPD pain since learning about it around the 12w mark. (I'd already been having pain from 7 friggin weeks on! Hence, my true-believer, evangelical status on this topic. ;) )