if I may chime in, MaZA has it exactly right. that is precisely the process I've used the 3 times I've done it on forces and it's worked wonders with minimal effort. you can do it for several days to varying levels of affect. this will work on any rubber midsole shoe that has yellowed from age.
sanding also can work, and it's actually helpful for any pesky spots you can't get to whiten from the gel. you have to use a fine grit, and you don't want to rub all the texture off the midsole. I have taken tough stains out with sandpaper without noticeably altering the texture of the midsole.
seaglow also works (I haven't tried, but i've seen before and after from other restorers), but it's a lot more expensive than the plus white, so there's no point in using it unless you already have extra laying around.
and of course, the rubber can yellow again, it's oxidization, all it takes is time. but at least you know what to do if you really want to get them white again.
also, I'm pretty sure the kicks Zach13g posted are ajko's, not af1's. correct me if i'm wrong.