We've used this method successfully, especially on roses:
Glycerine
This chemical replaces the water in the plant material, making the preserved plant supple and long-lasting. To use this method, the plant material needs to be gathered in a fully hydrated (non-wilted) state. Use two parts of water to one part of glycerine, making sure the water is luke warm for better mixing and faster absorption. Ordinary car antifreeze will work. If the autumn colors are showing, it may be too late to preserve them in glycerine.
Where leaves only are used, they should be submerged completely in the glycerine-water solution. Where leaves attached to stems are used, then only the stems are immersed into the solution. The time required for completing the preservation process varies, but expect two to perhaps three weeks before the glycerine solution reaches the leaf tips.
If the samples appear to wilt after removal from the solution, then hang them upside down so the glycerine will migrate to leaf and branch tips. For beginners who are looking for assured first-time satisfaction, (Source: Ronald C. Smith, Horticulturist)