I've been off line for a few days so I've only just read your post. Pressure sores are a nightmare for a number of reasons, including being extremely painful and an easy route to infection if the skin is broken. If you look around at disability suppliers you'll find a large variety of pressure relieving cushions from memory foam to egg box shaped – trouble is, you need to be able to try each kind for a few days to find out which one suits you. Perhaps the best bet would be your local occupational therapy department, who should be able to let you try out different ones, or perhaps even a local MS Society. Don't listen when an " expert" tells you which type is the best… only your bum can tell you that!
Since you can stand a bit, I'd recommend getting on to your feet as many times during the day as you can, preferably with a very good friend to rub your backside and get the blood circulating, (MS really lets you know who your friends are!).
My carers swear by an old remedy which they use if the skin is broken or in danger of breaking down: a bar of soap, moistened and rubbed directly over the area seems to work wonders by softening the skin and drying to form a barrier – don't rinse it off – better than any cream I've tried.
Finally I'd recommend taking care of your skin from the inside: a good diet is crucial, and keep hydrated.
I hope this helps,