Check for complete clutch release; put the pedal all the way to the floor and then raise it a half inch. You should be able to shift through all the gears that way when the vehicle is standing still; any hard shifting or the vehicle starts moving and the clutch likely isn't releasing all the way.
As far as an additive, try adding half a quart of Pennzoil synchromesh. It makes for incredibly smooth shifting.
What fluid are you using now? The Ford Motorcraft GL-4 manual trans gear oils are very good. I'm using the Motor craft conventional stuff in my Corolla and it shifts awesome.