Nothing. Here learn from me... will make me feel a little less dumb for wasting money on keyboard covers. =)
I bought the Moshi key guard, 2 clear crappy covers, a white cover, and a black cover. I am now not using any cover and happier for it. The covers all sucked. All of them, the expensive ones, and the cheap ones. My favourite was the white cover, but it faded very quickly (about a month).
So instead of getting a cover...
Every week, I use a cotton ball (well, they aren't ball shaped anymore, they are flat these days), and some hydrogen peroxide. Lightly clean each key. Do not press hard, you aren't buffing it, just cleaning.
Enjoy your naked keyboard. It types so nice =) You'll get some wear and tear sure, but honestly it is not bad at all.