Pizzagrenadier’s Wargaming Blog has a useful tutorial on how to rust weather metals with just three colors. No pigments or powders needed. The results look fabulous.
rust
Modeling Rusty Metal
Captain Richard has a tutorial on modeling rusty metal.
Painting Rust and Dust
Kevin’s Miniatures and Hobby Table has a good article on painting rust and dust effects.
Painting Rust Effects
The Man Cave has an article on painting rust effects. Lots of good techniques here.