while it is open or when it is closed? when it is closed I used to use a piece if felt the size of my keyboard, but now I have a leather one from smalldog.com. It is a lot thinner and makes the lid sit tighter then the felt. While open I have no idea. I couldn't handle anything on my screen, it would annoy the hell out of me
There are lay-ins from either smalldog or radtech that work well. They work to simply cover the keys from contacting the screen (especially keeping your finger oils off of it) which will help to avoid the bright squares some people have on their screens.
You should consider getting a laptop privacy screen from 3M. They are expensive, but if you can find one that is the right size they will both protect your screen and your information from peering eyes!