My GFR went from 54 to 74 in 2 years by cutting out the Aleve, Advil,
Ibuprofen (NSAIDS are really hard on the kidneys),
and following a low potasium, low phosphorus, low salt, low fat, diet. Phosphorus isn't usually listed in things. Dark colas and pretty much every pre-packaged food source.
My kidney doctor (nephrologist) basically said that making it easier on the kidney can allow it to function better. It will help the GFR stay the same and sometimes you can gain.