Here is an interesting post from the Chicago Classical Music blog. Bass vocalist Jonathan Miller gives some tips for thinking about intonation in the lower register, and his comments apply to the double bass as well:
First and foremost: sing squarely in the middle of the pitch. Most bass notes work well with this rule, partly because most bass notes (at least in the stuff I sing) are the “roots” of their chords. Everyone lines up on top of your note. It’s quite fun.
Read his complete post here.