Ok, here are som ways to dirty it up.
1. use pastel dust
this is the efect that you will get.
also on the bottom of the model theres some mud.
2. Apply mud, escpecially around D-day, the weather was bad.
3. give it some scratches. i use a #2 pencil. Put them on and around places youd expect wear and tear. (hatches for example)
4. put washes in recesed areas, to bring ou the detail.
5. dry brush to bring out the look of faded paint.
Not i know that i am forgeting some some, so maybe somebody else can fill you in.