Laptop owner Dwayne Melancon protects his laptop screen from scratches by keeping a piece of rubberized shelf liner cut to fit on the keyboard.
I ponied up the 20 bucks for an iSkin for my Powerbook, which stays on while I type (keeps crumbs out AND protects the screen), but Dwayne's method is way cheaper - though it does require putting the protector on and taking it off each time.
Lifehack: Laptop screen protection [Genuine Curiosity via lifehack.org]