Chris,
I made the hard learned lesson that the nice dusting layer of pigments can't be fixed permanently. I had dusted my M3 Lee nicely and added a layer of flat clear and then it was gone... So really the only way to do it is to know how to handle your model without touching the dusty parts, fix it to a base for example.
As for how to make it dusty, I mix my MIG pigments with (distilled) water and add liberally, then when dry brush off the excess with a rather stiff brush to make it more even. This will lead to the following effect:


Cheers!
Stefan