Quite an interesting question!
First things to try that pop in my mind (used very subtly) are chorus, harmonizer and distortion. That's to create a more misty, deeper "bedroom tone" :o). Maybe some EQ is needed as well, and advanced pitch-shifting, as Danny suggests, depending on the type of the voice.
Say, doubled track, the other one quite low volume, panned 20-30% off from the main voice, with distortion or something.
This all is just imagination, I've never tried anything like that for spoken voice, and it may be these simply don't work at all.
Compression, maybe parallel, should be tried, too.