How to Protect White Clothes from Yellowing

Like the title says... does anybody know? I bought this white dress that I think is chiffon or at least that's what it says on the sale page. Anything white that I've ever bought has always turned yellow and I'd like to prevent it if possible.

Since I'm writing about this I might as well ask... I also have another white sweater (polyester I think) that has yellow spots that do not change color one bit despite many soakings in a bleach solution as per what the internet instructed to do.... though I should point out that I didn't notice the stains right away and it's been through the dryer (if this matters... I don't know anything about cleaning)