You can strengthen magnets by placing them in water, stacking them on top of each other or recharging them. The method of making a magnet stronger depends on the type of magnet you have, explains HowStuffWorks.
To strengthen an iron bar magnet, fill a bowl or pan with water. Find a small item to float in the water and place the magnet on top of the item. The magnet starts to twirl in the water until it rests and points directly north/south. Remove the magnet from the water and keep it in the same position while placing it on a hard surface. Then, strike one end of the magnet with a hard object repeatedly. The force can loosen small magnetic domains in the bar.
You can recharge a magnet that has lost its strength by repeatedly rubbing it against a very strong magnet. The strong magnet's pull realigns the magnetic domains in the weaker magnet.
The last option to strengthen magnets is to stack all the weak magnets together. This can be difficult, as magnets attract in opposite directions. For this method to be effective, you must clamp or hold the magnets together so they all face the same direction.