What is the best way to keep towels white?

Use Hydrogen peroxide mixed with your laundry soap.

Janet's got it! Bleach will yellow whites over time. It also breaks down some fabrics. I use hydrogen peroxide with all my whites. For really tough stains you can use Rit Color Remover to get them out. BE CAREFUL! It removes all color. Not to be used regularly.

I use a half and half mixture of powdered dishwasher detergent and Borax. I put this in the machine before I add my items. Then add my detergent and a little clorox (or you can leave the clorox out). I let the items agitate in hot water for several minutes and then pause the washer and let the items soak as long as possible. Then just finish the cycle. My once really dingy sheets and towels now not only are wonderfully white, but smell like line dried clothes! It might take a few times to get the dingy out, but just keep using it. If I don't have time to let the wash soak, I still add the mixture and just wash as normal. I keep the Borax/dishwasher detergent in a gallon zip lock back with a measuring cup so it is always ready to go. ( I would use a pretty container, but don't have room--hence the zip lock bag.)

I use a whitener called Out White & Bright. Love it, leaves them looking brand new & takes very little.

drop denture tabs in toilet and tank. this might take a few tries leave in over night. let me know ifthis helps.