This article is from the Car Audio FAQ, by Ian D. Bjorhovde (firstname.lastname@example.org) with numerous contributions by others.
As far as router bits go, use only two flute carbide bits and make
several passes as opposed to one single pass. If possible, trace the
outline of what you intend to router and remove the majority of the
material with a jigsaw which will reduce the amount of material being
removed and will put less strain on the router as well, not to mention,
this will all reduce the amount of MDF dust.