Good fixation is important.  Most people use a mix of glutaraldehyde, which cross links proteins and other components together, and paraformaldehyde, which binds monovalently to lysines.  The fixative is buffered with usually cacodylate or phosphate buffers.  Delivery is by cardiac perfusion and should be done quickly.

Full details for mice and rats will be available for download soon

Of course Plant Fixation is more complicated by lignins etc.  People with a good method may want to share it through this page (comments below).

You can also use immersion fixed (drop fixed) tissue –  though results are often worse. A good method is  20160425_VS standardized sample Preparation Protocol