How to use magnets when hiding a cache
An excellent free source of small, powerful magnets is to take an old PC hard disk to pieces. The PC Doctor's excellent article Take a peek inside a hard drive! includes photos showing how to take the screws off, etc. You may be able to beg one or two broken drives from a PC store, or from a friendly geek.
The magnet tends to be arc (banana) shaped, typically glued to a similarly shaped but larger piece of steel. If you bend the piece of steel back and forth, you may be able to break the magnet loose from the glue without actually breaking the magnet. Or you can chip the magnet free with a hammer and chisel, and probably break the magnet in the process. You can also break the magnet into smaller pieces the same way deliberately. Beware of the very sharp edges which can result, which actually seem to be from the shiny covering rather than the main body of the magnet, and are easily filed down (with a non-steel file).
You can place the magnet(s) on the sticky side of camouflage duct tape, and then apply the duct tape to the outside of the cache container. There are two disadvantages to this approach: (1) multiple pieces of magnet will attract each other while you are applying the tape, and (2) sometimes the magnets will stay in place when the container is moved (either because the tape came off the container, or because the magnets tore through the tape).
Another approach is to stick the magnets inside the container. However, you can lose quite a bit of the magnetic force this way (depending on the material and thickness of the container), and it can be quite difficult to get most forms of glue to stick (especially to polyethylene-based plastic containers).