Secrets of the Pros
When coming up with names, I usually think of a word (usually an adjective) that describes the character or race. Then I tweak it to sound alien and voila, a new name is born.
So, for an underground slave race:
slarreks
(slave + barrack, as in slaves are kept in barracks)
It's not necessarily brilliant or hypercool, but it's only a slave race. Save the cool names for, well, cool races like demons.
If you wanted to get all old school, you could call them moldvays, after Tom Moldvay, co-author of the Slavelords series.