Just been reading about aromatics, and realised something funky about them, see .... http://en.wikipedia.org/wiki/Aromatics The ring structure lets the electrons whizz round the molecule. A magnetic field would be generated. However, if they were to be moved through a magnetic field, then that would set the electrons whizzing around. Ergo the molecule would probably move to a higher energy state and be more ready to react violently given the opportunity to do so.
Given that gasoline has a large mixture of aromatics in it, magnets on your fuel lines may, as the gasoline moves past them, boost up the molecular energy levels of the aromatics. Thus they would burn quicker.
On this thinking, the ideal positions of magnets would be in pairs on opposite sides of the fuel lines, arranged N-S so that a strong uniform field is formed between them. Bear in mind however, that this does actually produce a resistance to motion of the fuel flow and would place additional load on the pump, small though it may be.
This method may not produce additional results when used with fuel heating, because the heat energy raises the energy levels of all the fuel already. However, it should provide cold start benefits and allow greater efficiency during warmup still.
I'm wondering if the "use the south pole" wisdom may actually have a weak effect because most of us are in the northern hemisphere and using a southern pole against the line forms a weak flux with the earths natural northern field.
This method may be particularly effective with winter gas, and less effective to unnoticable with summer gas. This is because winter gas has higher proportions of aromatics, and summer gas has lower proportions.
Users of mothballs may notice an enhanced effect with magnets because napthalene is an aromatic.
well that's something else to play with and think about at any rate.