You Need
2 long steel spikes (30 cm)
1 plastic bucket full of standard steel nails (2.5 – 3.0 cm long)
1 neodymium magnet (2.5 cm diameter)*
What to Do
Secretly give one of the two volunteers the spike that has a neodymium magnet attached to the flat end, and tell that volunteer to 'hide' the magnet from the audience, by holding the spike with the magnet in a closed fist.
When the volunteers have a 'contest', there is a clear winner. Make a 'big deal' about the 'loser' not being able to lift any nails. (The winner must have a 'magnetic personality'.) Then, let them trade spikes and allow the audience to see that it was a 'trick'.