I was apparently too late for the glycerin, where you allow the leaves to suck up the glycerin, thereby dooming (or deeming immortal) the leaves to perpetual natural beauty. I thought laminating would give an artificial look. Dipping them in wax sounded, logically, difficult, as liquid wax is, necessarily, HOT and destructive to fragile tissues. So I decided just to press them in books for a start and then decide after how to continue.
Into the books they went, for about two weeks. Then I pulled them out and they looked wonderful. Flat, unbroken, not yet brittle. Seeing them like that, I decided go for what sounded like the most natural-looking result: wax them.
I went to the trouble of getting waxed paper (they seem to not have this in Germany) and proceeded to iron them between two pieces of waxed paper. With this method, you leave the leaf between the two sheets of waxed paper and cut around the leaf. This looked like crap: the colors were muted and the feeling of the leaf was lost. So I removed the paper and discovered that the wax left on the leaf from ironing was enough to preserve it (albeit for an unknown period of time). And since I was left with just a leaf, it looked completely natural, as if I had picked it directly from the tree.
However, since it was going to have to live a little as a costume, I wanted to give the leaf a bit more strength. After wandering around my house a few times looking for inspiration, I thought of the puzzle glue we had once used: it was a milky liquid that dried clear and flexible. Sounded perfect.
And it was. Quick and easy, with the natural-looking result I wanted. Everyone complimented our costumes - some people even thought our trees were REAL TREES!