Some permanent magnets are always strong, but some have a tendency to lose their attractive properties over time, or because of lots of use. Enough heat can even demagnetise a magnet. If you have some magnets and the pull is not as strong, you can remagnetize them. It is not a difficult process and your magnets are as good as new afterward.
Place a steel bar as a "keeper" across the poles of the magnet. It only needs to be long enough to span across both poles of the magnet. Keepers help keep magnets strong when they are properly magnetised and they help remagnetize magnets that have lost their attraction by aligning the field properly.
Coil 22 AWG wiring around the magnet and keeper several times. You can use other gauge wiring if you want, but it needs to be large enough to hold up to being hit with a hammer.
Build a battery pack. Line up four D-cell batteries in series (positive to negative, like in a large flashlight.) Run several pieces of tape along the length to hold them together in a row. Tape a wire with an alligator clip to each end.
Clip the alligator clips to each end of the coil and hit the coil with a small hammer. Disconnect the alligator clips.
You can use any battery assembly in this project. The higher the current, the stronger the magnet you create. For smaller magnets, find the north and south poles on a rare earth magnet. Touch the north pole of the rare earth magnet to the south pole of the permanent magnet. Repeat between the south pole of the rare earth magnet and the north pole of the other. You can also use a remagnetizer. Keep magnets strong by storing them with a keeper to keep the current in the magnetic field running in the correct direction and away from other magnetic fields.
When using higher current battery packs or batteries, like 6 or 12 volt car batteries, take extra safety measures: hold the coil with non-conductive tongs because the coil gets very hot and only touch the coil to solid metal terminals, rather than alligator clips. The risk of shock increases as you increase the current.