How could I cause metal to rust rapidly?
Here are factors that would speed up rusting:

Presence of water: make sure that the iron is wet.
Presence of oxygen: make sure the iron have access to air (the dissolved oxygen in water also works).
Presence of metal below iron in the metal reactivity series (see the picture below): usually copper is used. Tie copper to iron. Make sure that they are in contact. The larger the surface area of contact the better.
Temperature: make the iron as warm as possible.
Presence of salt: explained here. In short, salt increases the conductivity of water which makes the iron rusts faster, since rusting involves electron transfer.
Acidic environment: since rusting involves hydrogen ions ($\ce{H+}$), an acidic environment would increase the concentration of hydrogen ions, making the iron rust faster. However, this would dissolve the rust.
Presence of hydrogen peroxide: hydrogen peroxide ($\ce{H2O2}$) is an oxidizing agent which would not dissolve the rust:
$$\ce{3H2O2 + 2Fe -&gt; 3H2O + Fe2O3}$$

Metal reactivity series:

(source: bbc.co.uk)
Equation of rusting:

$\ce{Fe -&gt; Fe^3+ + 3e-}$
$\ce{O2 + 4H+ + 4e- -&gt; 2H2O}$
$\ce{2Fe^3+ + 3H2O -&gt; Fe2O3 + 6H+}$

More equations can be found here.
Overall equation:
$$\ce{4Fe + 3O2 -&gt;[H+,H2O] 2Fe2O3}$$