If it's specifically the coccyx, there are specialist coccyx cushions available, sometimes known as tailbone cushions, and some people absolutely worship these cushions after trying them. It's a big deal with French taxi and other professional drivers. The other advice is not to wear tight clothing around your hips and sit up straight, rather than slouch the pressure downwards.

Personally I think the A320 seats are fine, but I've got the padding.....