There's a lot of sense in these here posts. Picking up from what Kyte said, when I was in hospital with this recently, they explained a lot to me about pain management for SPD. The theory is that you have to train your 'pain gates' to be closed as much of the time as possible. In other words, with a condition like this the pain mechanisms get very trigger happy, and start to give out extra pain signals, but if you spend as much time out of pain as possible, then the less it hurts overall, and the quicker you recover. It sounded a bit counter-intuitive to me when they explained it, tbh, but they have been proved right in my case. If I take my medication like a good girl four times a day, and sit around practically all the time doing nothing, then I am better overall.
At the recommendation of the hospital consultant, I take quite high doses of codeine (60mg four times a day) and paracetemol at the same time to enhance the effect (500mg four times a day) with Oramorph for breakthrough pain, and use a TENS machine for a minimum of five hours a day as well. This is more extensive pain relief than GPs normally give (they tend to advise 30mg once a day at night), so I think it makes sense to try to get up to the hospital if things are very bad in your case (apart from anything else, taking opiates during pg needs to be carefully monitored). Bear in mind I normally have the pain threshold of an ox and rarely take tablets for anything, btw, so I have to be pretty convinced about something to pill pop like this as it goes against the grain.
Resting up a lot like Qally suggests and being very fussy with your pelvic care, and not being bullied into doing things that are bad for it, is very important as well.
They generally tell you to do lots of pelvic floor exercises and also specialist physio Pilates exercises to strengthen the area around the pelvis and promote stability. However from discussion on MN, a lot of us have found these exercises to be ineffective during pg. However they might help you get better quicker after the birth, IMO. If you work within your pain threshold, they certainly won't do you any harm.
A lot of people report huge improvements after osteopathy or chiropractic. I have seen a chiro a lot during the pg, and while in my case there was no magic cure, it does help with the pain (I really feel it if I have missed or delayed a session, for example), and also the baby happens to be positioned really well for the birth which may have something to do with the constant pelvic realignment she does for me - at this stage his brother was breech.
If you want to throw a bit of money at the problem, apart from shelling out for chiro/osteo, you might want to look at memory foam mattresses, Genii pg pillows, and satin pyjamas, all of which have been commented on favourably by MNetters with SPD. I happened to be signed up for a hypnobirthing course anyway, and I found this helped me panic about the SPD less as well, oddly enough.
The NHS can give you elbow crutches or a wheelchair if you need them (you might need to nag the Rehabilitation Department), and you can claim Disability Allowance for care and/or mobility if you get very bad. You can also apply for a Blue Disabled Parking Badge if necessary. But it may not come to that.
Birth wise they say you are likely to have an easy birth because they pelvis will probably expand nicely. Water birth is recommended because it supports all the joints - I wanted one of these anyway, but my consultant has also been encouraging it because she has seen good results with this in SPD cases. Flat on your back and forceps/ventouse ought to be avoided at all costs, apparently.