Scratch and sniff stickers are one of those novelty technologies -- they sound like such a good idea, but they never really caught on except in children's books and the occasional perfume strip in a magazine. Why shouldn't nasal information be just as important audio and visual information? Perhaps because no one has figured out how to encode language in smells...
Nonetheless, if you have kids you probably have a scratch and sniff book around the house. And even if the book is 20 years old, it still works! The reason the stickers last so long is because of the microencapsulation technology used to create them. The basic idea behind scratch and sniff is to take the aroma-generating chemical and encapsulate it in gelatin or plastic spheres that are incredibly small -- on the order of a few microns in diameter. When you scratch the sticker you rupture some of these spheres and release the smell. The smell is essentially held in millions of tiny bottles, and you break a few of the bottles every time you scratch the sticker. The tiny bottles preserve the fragrance for years.
It turns out that the microencapsulation technology used in scratch and sniff was first developed to create carbonless copy paper. The top sheet of paper is coated with microcapsules containing a colorless ink. .When you write on the paper it breaks the capsules and releases the ink. The ink mixes with a developer chemical on the next sheet of paper to create a dark color!
Here are several interesting links: