I have also had trouble reducing graininess. I use Blender to model buildings and graininess (noise) is always worse on interior scenes.
I don't believe there is one simple answer.
Certainly increasing the number of samples (to say, 1000) will help.
If your scene includes glass you should ensure you are modelling it correctly (so that Shadow rays see a pure transparent shader).
I have always found graininess to be most severe when modelling an interior lit from outside (through apertures). My intuitive understanding of what is going on (which might not be correct) is that the randomly chosen light paths 'bouncing' off diffuse materials have trouble 'finding' the light sources. One pixel will find a light source, a neighbouring pixel might not. Increasing the number of samples gradually smooths this out.
There are other things you can do also (caustics, clamping, portals) but I am afraid I don't know enough to advise you reliably on those