You can strengthen magnets by placing them inwater, stacking them on top of each other or recharging them. The method of making amagnet stronger depends on the type of magnet you have. 

To strengthen an iron bar magnet, fill a bowl or panwith water. Find a small item to float in the water and place the magnet on topof the item. The magnet starts to twirl in the water until it rests and pointsdirectly north/south. Remove the magnet from the water and keep it in the sameposition while placing it on a hard surface. Then, strike one end of the magnetwith a hard object repeatedly. The force can loosen small magnetic domains inthe bar. 

You can recharge a magnet that has lost its strengthby repeatedly rubbing it against a very strong magnet. The strong magnet's pullrealigns the magnetic domains in the weaker magnet.

The last option to strengthen magnets is to stack allthe weak magnets together. This can be difficult, as magnets attract inopposite directions. For this method to be effective, you must clamp or holdthe magnets together so they all face the same direction.