I use a Sansa Fuze for my audiobooks. It's 4GB one supplemented with a 4GB minicard.
For the Fuze, I need to make sure that the ID tag of the track shows as "audiobook", then it automatically gets filed in the Audiobook segment of the Fuze rather than getting mixed up with the music tracks.
I make sure that the ID tag shows the track number and I make sure that the track file name is also numbered sequentially, e.g. "City of Ghosts Part 1", "City of Ghosts Part 2", or "01 City of Ghosts", etc.
The books that I download from Audible via Audible Manager automatically go into the Audibles portion of the Audiobook section. The others I just drag and drop into the Audiobook section. It is essential, though, that you have the audiobooks' ID tags completed otherwise you wind up with files called "Unknown".
Sansa has a pretty good forum of their own for their various devices; you might want to check that out.