hot soapy water or autoglym interior clean is really good and a sponge or plenty of clean cloths just fill a spray bottle with either and spray on the dash and wipe away until you have got the dirt off. then dry and apply either a matt or high gloss dashboard / plastic spray as i said i only use auto glym products.
Make sure you charge enough to cover the time spent as its not a 2 min job i tend to stay away from the really really dirty jobs
for getting in the vents and cracks of the dash i find that cotton bud sticks are really useful just dont put too much water on the dash for obvious reasons.
what sort of floor covering is in there is it carpet or rubber matting?