The Invisible Shield comes with a spray on adhesive which you use to stick down the shield. I had originally put it on my Powerbook after replacing the original top case due to the black aluminum pocking that some people experience.
Unfortunately, while the Invisible Shield did protect the Powerbook, the Invisible Shield itself did get a bit of a yellowish tarnish.
Once I bought my MacBook Pro, I removed the Invisible Shield from the Powerbook. The adhesive is strong, but it came off clean. I just used one of the mac wipe clothes to remove some dirt that had gotten in next to the edges of the Invisible Shield and it was as good as new.