First start by figuring out where you want rust. (duh)
Now I do several places on the model to help sell the effect.
Next, take some Calthan (or equivalent) Brown, and thin it down with water, you want it almost like a glaze. I would say 35% paint 65% water. Paint this mixture where you want the rust to go.
Next, using a stipple brush, or drybrush, dab some Orange paint in the middle of your brown area. the more orange you use, the more the item looks rusted. (Note: too much ,and the item would fall apart. So be careful)
And that's it! Quick, easy rust, that will really help you achieve a "realistic" feel for your army.