As long as you use paraformaldehyde or formalin, your GFP will maintain
fluorescence.  However, avoid all solvents (you cannot paraffin process!)
in a further staining, etc, frozens will be necessary or thick vibratome type
sections.  

GFP can be dicey to coverslip and view also.  GFP filters on UV scope are
superior, and mounting medias (aqueous) caused GFP to fade in our particular
instance.

Gayle Callis