Mike,
The system of weathering I use is to apply Nouvel Pastels. The advantage over Tamiya powders is they come in a variety of colors. Great if you want to have yellow rust or more serious dark brown. I scrape it into a soda cap and apply it dry in some cases with a micro brush. Take off the excess with a 'Q' tip, sometimes dry, sometimes moist.
The other use is to add a bit of water and applying as a water color. The best part of both of these techniques is any mistake can be easily cleaned up with water. For most of my projects the pastels are applied to flat paint so the pastel has some traction. Experiment with them and I think you'll be happy.
Hope that helps.