Like Kath said - get a tailbone cushion! I found a few good ones on eBay and got a couple of different types. I've been real happy with the foam cushion that has the strategic cutout notch below the tailbone. Was okay with the hydro-cushion until it dumped its water into the seat cushion and went flat. Took awhile to get the water out of the chair!
There are also some inflatable donut types that should work well. The trick is to get support on the cheeks without sitting on the tailbone.
The only way this is going to go away is if you can repad the area with fat, but that's not why we're here! Live with it, love it for what it means!!
CT