I asked this question a while back.
The best way seems to be to submerge them in a half vegetable glycerin, half water mixture for 6 or 7 days. This requires that they still have their stems, so you must pluck them from the tree. The glycerin is absorbed into the leaf’s cells and preserves it’s color in addition to keeping it naturally flexible.
I call it the best because it preserves from the inside out, rather than sealing the leaf inside of a covering. I think the flexibility factor is a huge perk.
Not sure if this works with green leaves though… I will have to try it and get back to you.