What is your helmet made of?
Indeed if it is made of smooth plastic, you can wipe it with a damp cloth or a microfiber one. Be careful not to use any solvents that could damage the material.
However if you helmet is covered with velvet, you can brush the dust with a soft brush and clean the stains with a damp cloth.
I don't use any special products, clear water is what best work for me, the simpler, the better!
Hope it helps :)