How to preserve leaves between sampling (collection) and analysis?
You may want to see:

Mark W. Chase, Harold H. Hills; Taxon, Vol. 40, No. 2,  1991, pp. 215-220: 'Silica Gel: An Ideal Material for Field Preservation of Leaf Samples for DNA Studies'

Summarized, the process would be the following:


Place the leaves inside ziplock plastic bags;
Add ten times (minium) the weight of the leaves in silica gel;
After 12 hours (after which the degradation of the biological material is more likely), check the leaves by bending them: if they break cleanly, the leaf is dry;
Store in the ziplock bag, inside a box with an hermetic lid;
Refrigerate (if possible).


Note: Freezing by submersion in liquid nitrogen is also a viable approach, yet might be less practical