There are saddles that are split in the back to take all pressure off the tailbone area. I ride with a gal who injured her tailbone badly in a bike accident years ago, and she needs this type of saddle.

Here's just one example:

http://ridingbicycles.com/itemdetails.cfm?LibId=29722

Warning, tho, a lot of these saddles cost more than the bike you are are currently riding!