I pour off about half my sourdough every night and feed it with 1/2 cup flour mixed in 1/2 cup water. If I am planning on baking the next day, I don't pour off so I am sure to have plenty. If you think your sourdough has died, don't pour it out. Just keep pouring off and feeding it. It will come around. Remember to use pure water to feed your sourdough, because chemicals in tap water can kill it.
For more help, go to Sourdough Home or Wild Yeast Blog. (my favorite bread baking website!)