I've said it before and I'll say it again, after a nod to those who have mentioned attention to general health, drinking good old plain room temperature water in large quantities will help take care of your voice, ease congestion, etc. If you are sick with a cold or flu you need to drink extra water anyway, as well as rest. I also find that prolonged periods of silence and basic (Zen or Vipassana) meditation do wonders for the voice, sick or well -- I sing my absolute best when I've returned from a weeklong silent retreat.

Cheers!

Sharyn