There are several methods for doing this. One of the easiest is to used black,brown, and grey spraypaint and lightly mist over the helmet randomly creating a dingy, dirty look. This helps give the scratches that 'not so fresh' look.
You can also used crushed black, brown, and grey chalk pastels and dry brush them onto the helmet. You will need to give it another coat of dullcoat after you apply these to seal them onto the helmet.
Then there is the always tried and true dark 'wash' that can be applied. While I have never used this method, I believe you can find an explanation of how to accomplish this in Rogue Studios 'Paint List' Thread.