I've had some success with some tree leaves by using a solution of glycerin and water. Leaves will turn shades of brown, but they will also be supple and have more natural form. However, having said that, you should know that my last effort at preserving walnut leaves failed. (Perhaps they were too young. They were also the only leaves without insect holes and other blemishes.) I bought my last glycerin in the drug department of Walmart, but when I asked for it yesterday, they said they didn't carry it. (Glycerin is a gooey substance often used in hand creams, and it's full of moisture.) If you use the stems with blossoms and leaves, your blossoms will also turn the same shade of brown. Good luck if you try this!