I've been using two techniques in conjunction with the MMP weathering powders. A little cap full of Future mixed with the powder color of your choice, and applied with a old brush. Works like a champ.
Second technique, is to take the same powders, mix them with rubbing alcohol, and spray through the airbrush. The mix wants to separate immediately, so you have to keep it stirred and spray only small amounts at one time. The effects are perfect dusting, that nice thin layer of dirt or mud, and ground-in dust in high traffic areas like hatches and front glacis plates. It also works real well if you want to deposit mud or dirt into crevices like road wheels, on tracks, etc.. It will get quite dark when you apply it, but once the rubbing alcohol evaporates away, it returns to the original color. Apply it a bit heavier than you like, so that once you've dull-coated it, there's something left, as the clear flat tends to negate much of the effect.
Jeff