descending indexes

I am attempting to port an application that makes extensive use of being
able to scroll forwards or backwards through a large database from a
given, user-defined point  where the key is a CHAR(15) user-defined
field, arbitrarily & frequently inserted & deleted.  Creating an index
on the field speeds up the scroll forwards, but I'm having to jump
through hoops to scroll backwards.  I have searched and cannot find any
reference to a simple method to find the previous key using the index.
Any suggestions?

steve