I keep my keyboard clean and protected from spills with a Moshi keyboard cover. They’re very thin, very flexible, and highly durable. In my experience the covers last for about two years.
The covers accumulate oils from your hands, but if you keep a little microfiber towel handy, that cleans off most of the oil. I wash the cover once a day or so. They’re a bit hard to clean; I use foaming soap with warm water. Lather, rinse, repeat, and then let it drip dry.
The moshi covers are for MacBooks and most other Apple products (ncluding the previously reviewed Bluetooth Keyboard. I’m guessing that covers exist for PC laptops, but I don’t know who makes them, or which ones are good (Note: if you have a suggestion for a good brand for covering PC keyboards let me know and I’ll update the post.–OH). The thickness/flexibility of the covers is very important. One of the brands for Mac computers was very thick and felt yucky to type on. I recommend trying a cover before you buy it if you can.
[Update: I mistakenly described the keyboard cover as silicone when it is actually a thermoplastic urethane. Sorry for the confusion. -- OH]