I am drying foliage right now using half glycerin and half water. If you are doing not so woody stems then it could be 40% glycerin and 60% water. Make sure the water is real hot when mixing with the glycerin before putting the leaves in so it is incorporated well.

oh thank you. A stupid question for you. Where would I get glycerin?

I picked up glycerin locally at a drug store. You can also try the glycerin used in soap making as long as it's in liquid form. I've recently used glycerin to preserve white pine branches, dog wood stems, ferns, and other interesting leaves I've stumbled upon. Hope this helps.