Some of the seedlings that I start indoors collapse and die shortly after germination. Why?

FAQ
Question: 

Some of the seedlings that I start indoors collapse and die shortly after germination. Why?

Answer: 

Damping-off is probably responsible for the collapse and death of your seedlings. Damping-off is caused by several different fungi. Environmental conditions associated with damping-off are poorly drained potting soil, overcrowding, and excessive watering. Damping-off can be prevented by using clean containers, a sterile, well-drained potting mix, and by following good cultural practices. Wash previously used containers in soapy water, then disinfect by dipping them in a solution containing one part chlorine bleach and nine parts water. Sow seeds thinly to avoid overcrowding. Flower and vegetable seeds need an evenly moist potting mix for good germination. After germination, allow the potting soil to dry somewhat between waterings.