My older POTF2 Stormtrooper has yellowed quite a bit. I consider him my "dirty" stormtrooper though
All I can suggest is that if you want to display them, put them in some place where they won't get any direct sunlight. Shield them from it in the display or put the display in a place where the sun won't get.
Maybe you can put some kind of overhang on your display, something kind of like an awning on a building? Or maybe stick them underneath a hanging ship in a strategic position- with something big like a gunship or the Falcon it'd probably block the sunlight okay. Just make sure it doesn't fall or you'll get some pancake clones
If you want to keep them stored though, I imagine they'd be fine in baggies in a box or some such.
In the worst case scenario that they turn yellow, you could maybe paint them white if you had the skill. Or you could even do some customizing things, like make some into camoflauge Kashyyyk troops.
If you have a huge number of them, you might experiment with a few at a time- keep some out in a place and if over a period of waiting time they start turning yellow, try something else, and so on.