Also, since navicular affects the deep digital flexor tendon, try keeping the concussive surfaces and strenuous exercising to a minimum, if possible, since it can accelerate the symptoms of navicular.
Try the bisphosphonates Tildran or Osphos. They're relatively new, but are proven to help relieve discomfort in navicular horses.
Previcox! Works wonders for a bunch of rescue horses I work with. Anything from ringbone, arthritis, navicular, tendon issues, etc.