Use a hammer or meat mallet to smash the ends of the branches - this will increase the area that will be absorbing the glycerin mixture.
Combine two parts boiling water to one part vegetable glycerin in a jar or vase. Once the mixture has cooled to room temperature add your eucalyptus. In about two weeks you'll notice that the leaves have softened, and turned darker in color.
Trim the ends if needed and enjoy!
Notes
I used 2 cups of water and 1 cup of glycerin for this post - you may need to adjust quantities based on the size of your jar or vase.
Much like forcing branches, you'll get the best results preserving eucalyptus if you smash the ends of the branches to increase the area that will be absorbing the glycerin mixture.
While the branches are preserving try to keep them out of direct sunlight if you can.