How can I strengthen a magnet?

Strong permanent magnets are manufactured with high-powered electrical equipment that is not found in the home.

You could perhaps make the item into an electromagnet by wrapping it with lots of wire; see a basic science textbook and/or a good encyclopedia article about them---

http://en.wikipedia.org/wiki/Electromagnet

Some products use magnetism in their construction. Magnetic earrings or aquarium cleaning products, for example, often employ two strong magnets to hold parts of the product together, allowing them to cling to the ear or slide up and down an aquarium wall. When permanent magnets become too weak to function, something must be done. Fortunately, there are a few ways to make your magnets stronger.Test your magnet. Place it on a vertical metallic surface, if it starts to slide down the metal, it is weak. Try to pull it off--if there is very little resistance, it's weak. The harder it is to pull off the metal, the stronger the magnetism. If you have a weak magnet, it has either always been weak or has steadily weakened over time. If it has always been weak, there is very little you can do. Discard it and focus on strengthening any magnets that have grown weak over time.

Strengthen A Magnet

It needs to be rezapped. Either replace it with a new one of the same type, or return it to the maker for the fix. Converting a permanent magnet to an electro-magnet probably won't work very well, because magnetic material, oddly enough, does not conduct magnetism well. Electromagnets are made of soft iron or plain steel.