bhai jan , shampoo must hai if u wish to keep ur car skin swril/ secratch free while washing.
its true if car skin properly waxed than u can rid off dust by simple water pressure but still i dont suggest this method.
if car is waxed than rinse car with water, spray car shampoo solution by spray bottle and than wash with presure water.
i follow samefast method however double bucket method much safe.
another fastest method would be foam lance but i doubt its avilability here in pakistan :S