Paper turns yellow if it's not acid free and exposed to air and sunlight. Wrapping them in plastic to prevent both elements is a common practise, but I just don't see the point. I'm aging, and so is my collection; I think I had to come to terms with that, and although I feel sad to see the paper weakening and turning yellow, at least there is solance that you've read and read them and that the stories are part of who you are.
I have been very interested to hear how you guys store your collections! I have a box full of manga that I brought and didn't like, or got bored with and never finished collecting. It's not a big box, in fact there is only a dozen or so titles. One day I might ebay them, but by then who would want a bunch of aged books? The hoarder in me can justify keeping anything lol.