I remember reading about using 100 proof clear alchohol such as gin or vodka. Supposedly you soak the material (acorns, pine needles, oak leaves, etc.) in the booze for a couple of weeks, strain out the material, and store the liquid. The article said that the good stuff (100 proof vs 80 proof) has no smell once it dries but the scent absorbed from the other material remains.

I've never tried it, but if you do, please let us know how it works.