treat before you store, or as you store. Chlorine bleach, unscented. So many drops per gallon. I've even used no additional bleach before and the water has never smelled or tasted bad after 6 months.

Again, the water I store is for general usage, if I need to use it for cooking or drinking, I'll most likely boil it before use if possible just to be sure.

info on treating water here:

http://firstaid.about.com/od/emergen...reat_Water.htm