Many readers have asked for plans to build a pulse magnetizer. We do not have these plans as the voltages and electrical currents involved can be dangerous. However, a good starting point and resource on magnetism is Arnold Engineering. They have a lot of resources that might help, including a list of magnetics suppliers where you can buy a pulse magnetizer.
A much easier and safer solution is to find a very powerful neodymium magnet and use it to magnetize things instead of a pulse magnetizer. These are surprisingly easy to obtain as they can be found in most computer hard drives. Just locate a discarded PC someplace with an old 400MB hard drive or some other relatively small capacity drive not used any more. Open it up and discover the powerful magnet inside. It will be obvious because your screwdriver will be strongly attracted to it (and will probably get magnetized by it)! The bigger the magnet you can find, the more power it will have to magnetize things, so look for a really big old toaster-sized 10MB hard-drive someplace.
In university or industrial labs, magnets are usually magnetized or remagnetized in "pulse magnetizers," which are electromagnets (coils of wire) powered by a large pulse of electric current. If you don't have access to such equipment in your community, the best you can do is to try to remagnetize them with a powerful permanent magnet, such as a large "rare earth" or "neodymium" magnet. If you can buy or borrow one, find out which faces the poles are on, and touch one pole of the rare earth magnet to the pole of your magnet that is attracted to it. Then touch the opposite pole of the rare earth magnet to the opposite pole of your magnet. This should improve the magnet's power.
Your magnets that have lost their power are probably steel magnets. They demagnetize (lose their attractive and repulsive power) rather easily. Modern magnets such as alnico magnets, ferrite (ceramic) magnets, and rare earth or neodymium magnets seldom lose much of their power. The cheapest are the ferrite (ceramic) magnets, but they will retain their magnetization far better than steel magnets, which are no longer used in industry for that reason.
David Rees asks (Aug 17, 2003): How can one build a magnetizer to re-magnetize ceramic magnets? In other words are there plans available?
Answer: Standard lab magnetizers use an electromagnet coil connected to bank of capacitors. You charge the capacitors and then discharge them through the coil, producing a brief but high magnetic field in the coil. Should be easy to build, although it may take a bit of trial and error to determine how many capacitors will deliver sufficient current to magnetize a ceramic magnet. The more turns in the electromagnet, the less current you will need, and it's good to keep the current low to avoid overheating the coil.
Far simpler, many ceramic magnets can be at substantially remagnetized simply by touching a pole face to the pole of a large neodymium magnet.
Dr. Livingston answers: If you have two disc-shaped neodymium magnets magnetized so that the faces of the discs are the north and south poles, and you join them together face-to-face (north pole to south pole), that will produce a moderate increase in the strength of the magnetic field emitted from the poles. Adding more magnets of similar shape to those two will increase the emitted field a bit more, but in a decreasing fashion, so that each new magnet will produce a smaller and smaller increase. Ten magnets in a row won't produce much more field than four or five.
As I understand the second question, you want to use a group of neodymium magnets to strengthen a ceramic (ferrite) magnet. If the ceramic magnet is not already fully magnetized, contact with a strong neodymium magnet in the proper orientation can further magnetize it.
You can get a book for less than US$10 from Lindsay Publications that explains how to make permanent magnets. I built one then beefed it up. It has been fun however it cost about $75 and requiored some machine shop equipment. For those with lesser needs, a rare earth magnet works very well to magnetize small magnets, especialy the ceramic type.