How can I prevent my seedlings from damping off?

I started hundreds of seedlings a few weeks ago and now some of them are damping off and dying.How can I prevent this from happening.Before planting I washed the trays and cleaned the tools with a mild bleach solution.
I even made a concoction to keep that from happening that I got from a gardening book.I am not sure what more I can do.Does anyone have a solution for me?