You've acquired a good chef's knife, you're using it almost daily to make tasty dinners for the family, and it's stored in a nice knife rack or a magnet for safekeeping. So why stop there? Keeping that knife's edge fine will make cooking not only safer but, let's face it, much more fun. Whether you've spent £150 on a high-end knife or under a tenner on a dinky paring knife, keeping it sharp is crucial. 

Begin with your lower-grit stone. Place the heel of your knife on the far edge of the stone, holding the blade gently but firmly with both hands at a 15- to 20-degree angle. Using even pressure, slowly drag the knife over the stone toward you down the length of the stone while simultaneously moving the knife such that the contact point moves toward the tip of the blade.
To use Al's method, take a black felt pen and shade in the bevel of the knife. Then take two strokes on the stone and examine the edge. If you have maintained the proper angle then all the black will be gone. If you see black on the top of the edge it means you are holding the back of the knife too far from the stone. If there is black on the bottom of the edge but the top is clean then you are laying the knife too flat on the stone and you need to raise it a bit. Repaint the edge and try it again. Once you discover what the right angle looks like then just maintain that.

If you do not remove enough metal to create a new edge, you will leave some of the dull edge in place. A dull blade (or a blade with dull spots or nicks) will reflect light from the very edge of the blade. A razor sharp knife edge will not show "bright spots" when you hold it blade up under a bright light. You will need to remove enough material from the sides of the bevel so that the edge stops reflecting light.
In summary: Peter Nowlan is a professional knife sharpener based in Halifax (Canada) and he recommends the KnifePlanet Sharpening Stone Set, a beginners and intermediate kit that includes 4 sharpening grits: 400/1000, 3000/8000, a bamboo base and the KnifePlanet Flattening Stone. The Japanese Naniwa 3-stone combination is also a great (and more expensive) choice, ideal for professionals and more advanced sharpeners: the Naniwa stones are slightly bigger compared to KnifePlanet’s. In both cases, a coarse, medium and fine grit combination is very effective to sharpen and refine the edge:
Diamond plates are available in various plate sizes (from credit card to bench plate size) and grades of grit. A coarser grit is used to remove larger amounts of metal more rapidly, such as when forming an edge or restoring a damaged edge. A finer grit is used to remove the scratches of larger grits and to refine an edge. There are two-sided plates with each side coated with a different grit.[14]