Van Dykes Taxidermy supply sells a product called Knoblochs plant preservative that is used to soak into the plant. Or they sell artificial plants like sagebrush that is pretty realistic. We used to use a preservative called "Never Say Dye" but I'm not sure if it is still made.
There's a place in Montana called Wild Things Preserved (wildthingspreserved.com) that preserves live plants, including sagebrush. I have used their products and would highly recommend them.