first, i don't work for owc, so this is not a commercial ad. but i purchased a thin leather square from them exactly for this purpose. i use a silicone key guard, and i find that it pops off and smudges my screen when transporting. this leather thing is suade on the keyboard side, and highly polished on the screen side. the good thing is that since it's leather, it's practically indestructable. i've read about plastic ones that can double as a mouse pad, but it comes in a tube, so it takes forever to get it not to curl.
i should add that i've got a anti-reflective, HR screen, so oil deposit is a serious issue. glossy people can easily get a wet cloth and wipe it down, but mine, no.
my $0.02