Keeping an expensive knife as sharp as it was when you first took it out of its box is an ongoing process.
What is it? Knife sharpeners, if used correctly, do as their name suggests. Sharpening can be done against almost anything that's slightly abrasive, including the bottom of a stone bowl but there are purpose-made knife sharpeners, such as whetstones, sharpening steels, rotating grinding wheels and products such as the Chef's Choice, which uses magnets to guide the knife against abrasive edges.
How to use: home cooks need only two sharpeners: a whetstone and steel. The whetstone, which some people soak in water or moisten with oil before use, usually has two sides: both are abrasive but one side is coarser. The rough side is used for sharpening a dull blade; the other side smooths the edge. As long as you maintain the sharpness of your knives, it shouldn't be necessary to use the whetstone more than two to four times a year. The sharpening steel should be used every time you use your knife because it will maintain its sharpness. It won't sharpen a dull edge, though.
What to look for: the rougher side of the whetstone should be coarse enough to sharpen the knife but not shave off too much metal. The other side should be smooth enough to polish the blade. Bigger is better (up to a point) because it can be hard to sharpen on a smaller stone. Steels tend to be round, but some are oval or have a flat edge; the sharpening part can be smooth or grooved, coated in fine diamond dust or made of ceramic. Whichever type you choose, the steel should be at least as long as your longest knife - sharpening a long knife on a short steel is dangerous.
What else? Fold a damp kitchen towel and put it under the whetstone before use; this prevents it from moving.
Where to buy: PanHandler (tel: 2523 1672), Pantry Magic (tel: 2501 0988), the cookware shops on Shanghai Street (take exit C of the Yau Ma Tei MTR station) or the kitchenware sections of Wing On (tel: 2852 1888) and Sogo (tel: 2833 8338).