I use what ever works:
1 Good ol' ground up dirt (as mentioned) microwaved to kill the little crawlies and molds, mixed with white glue (wet mud), with acrylic paint mixed to match its dry color (dried mud), sprinkled over a coat of the same acrylic paint (dried, compacted or dusted soil).
2 A mixture of putty and some sort of grit.
3 Pastels (as mentioned), which can also be 'powdered' or brushed on.
I love messy, dirty tanks (just like they REALLY look). Light weathering is great for 'in garrison', 'on parade' or New into battle, but not for a well used (even a couple of days out) vehicle. Unfortunately, the current 'doctrine' is for weathering that's so light it makes the vehicle look as if it's in a museum or something. I DO do light weathering when I want to show off a cool camo scheme or something, but usually LOVE HEAVY, VERY HEAVY weathering.
Ron