Make the determination over whether the weak magnet in your possession has become weak over time, or if it has always been a weak magnet. When a magnet is created, the electrons inside of it are aligned and stuck so that they point in a north/south direction (the magnet's polarity). The number of electrons that line up in this configuration determine how strong a magnet is--the more electrons, the stronger the attraction.

When a magnet is first made, it has the highest number of electrons pointing in this direction, therefore being as strong as it can ever get. If a magnet has been weak since you bought it, it likely has been weak since production and there is nothing you can do--you should discard the magnet.

Place your weak magnet within the magnetic field of a much stronger magnet. Setting it right next to the magnet will produce the best result. The stronger magnet will actually help realign electrons that have spun out of axis since it's creation.

Stroke the weak magnet with your larger, stronger magnet. Stroke it in the direction of one polarity to the other (or from one side of the magnet--the side that attracts other another magnet--to the other side of the magnet--the side that repels that same magnet). This will further align moved electrons.

Place both the magnets next to each other inside a freezer. Heat, radiation and electricity all play a part in diminishing a magnet's magnetic field. They do this by speeding up electrons' movements, causing them to shift out of place. Cold slows down magnets and holds them in place. Placing a larger magnet next to a weak magnet in a freezer will actually be a culmination of all these processes, causing your magnet to increase in strength. Take out the magnet and test its strength.