It's a known recommendation that is often given as a method to get rid of yellowing on bedding: rub in a paste made of soda crystals and water.
I always felt skeptical about this but tried it out the other day and it worked SO well. I shook over the soda crystals, dribbled on some water, and massaged this into the fabric before leaving an hour.
Just felt the need to share as yellowing has bugged me for ages and I didn't think there would be such an easy way out of it!