It depends a lot on what the surface underneath the rust looks like. If you have painted armor, you'll want to apply a chipping effect to it first before adding a rust effect-- basically, you do that by blocking out the chipped area with black, filling it with silver but leaving a black outline, then outlining the inside of the silver area with a lighter silver. Then, you can stipple a dark brown over it, followed by a mix of orange and brown, and then straight orange. If you've already got a metallic background, then a wash of Devlan Mud to shade it rather than black actually helps "age" the background more, and then you can just stipple the rust effect as described above.
For aging brass and bronze, you basically do the same thing as with silver, but instead of orange, you'll use a teal. Start with a very dark green (like Dark Angels Green), then mix in about equal parts Hawk Turquoise, then stipple Hawk Turquoise on its own.
