One trick I learned for hard stones like an India or Arkansas stone is to occasionally lap the stone with loose silicon carbide powder like people use in rock polishing machines or for sand blasting. This is not flattening but really only "scuffing" the surface of an already flat stone to break the glaze that builds up as you use them. What happens is that over time the stone becomes worn smooth in use and does not cut as aggressively as when it was new. People refer to this as "breaking in" sometimes, but eventually the stone gets smoother and smoother and then people will say that their stones "cut so slow".
If the stone is oily, blast the crud off of it with brake cleaner. Then, I will take a little 100 grit SIC powder and put it on a piece of scrap glass or a flat piece of steel. I then lap the stone using a figure 8 pattern for about 30 seconds or a minute using the weight of the stone only. If you bear down on it, you will only crush the grit up and make it less effective. You can use sandpaper or even a flat section of your drive way or a cinder block to flatten a stone like this but having two fixed abrasives rubbing against one another tends to 'polish" both surfaces whereas the loose SIC powder does a better job for lapping.
Incidentally, this is why people say water stones cut faster than oil stones. Most water stones have a loose bond and constantly break down in use, always releasing fresh abrasive. Many oil stones are the hard type and get clogged and glazed because people do not know how to maintain them. I sure didn't.
You can get SIC powder off of ebay which is where I got mine. A pound of it will last forever.
If you have the type of stone that releases slurry in use, you don't need to do this process as it is always releasing fresh abrasive. That is the advantage of this type of stone, the disadvantage being they dish and wear out fast. You can just flatten them and they will be good to go.
