I found a couple interesting galleries, here, and here. It's a tricky matter. The vast majority are due to either embalming or natural mummification. A few, such as St. Bernadette of Lourdes, shown above, pretty much defy explanation.
That said, few of the faces you'll see in the galleries are actually the face of the saint. They're the result of the face being covered with wax, or silver masks, long ago, though still long after their deaths. Yes, that includes St. Bernadette, whose face was covered with wax when decay started to show. If you want to see what incorruptibility really means, have a look at St. Zita.