I still have a 2 leaves from a plant I grew 9 years ago, and they're still as green as the day I took them from the plant.
All I did was sandwich them between the pages of a book, the book has slightly glossy pages, so this meant they didn't absorb too much of the moisture from the leaves too quickly.
However I don't know how long they will stay this way if they were to be displayed and exposed to light. Some form of lacquer will help protect them from the air exposure, but it would be best to keep them out of any direct sunlight when you do display them.