Sharpening stones are the best way to keep your knife blades sharp and in the best possible condition. But sharpening stones also need maintenance, since they tend to dish out over time and give you a rounded blade edge.
A sharpening stone will perform best when it's flat, and there are a couple of different techniques used to keep that even surface. The easiest method is to apply some 100-grit sandpaper to 1cm of glass with some adhesive, then run your stone over the sandpaper it until it's flat. If your sharpening stone came with a flattening plate, however, you can use that to remove the high spots on each end of your stone with just a few passes.
Check out Wood Magazine for more detail on keeping that sharpening stone in shape.
How to Flatten Sharpening Stones [Wood Magazine]