I'm definately no expert on the matter, as the above poster has said, they arn't bred in captivity much. However I would suggest starting with a rather large enclosure; they are large animals as you know. Attempting a breeding pairing in a cramped enclosure might just end in a stressfull confrontation.
The only other suggestion I can think, and I'm not sure if it relates to their breeding behavior, but in their native habitat, there is a lot of mud. Muddy banks, muddy bottoms. If you arn't successfull in your breeding attempts, recreating this sort of environment might be something to try.
Ian