There are many techniques to doing rust that range from the simple to complex, although I have only attempted one, I know of several methods of doing rust.
A simply one is stippling (or a very light dry brush can work) a mix of brown and orange onto the area you want rusty, and slowly increase the amount of orange.
Or if you want a more tarnished look, devlan mud and gurphoon (sp?) sepia are saviors, give it a few thin washes of that stuff to get a grubby (but not quite rusty effect)
And if you want to splash some cash, forge world do a range of weathering powders which you can mix a little with water and paint onto the area you want rusty
IIRC.
http://www.forgeworld.co.uk/Modelling-Supplies
There was also a
WD a while back with tips on rust effects, I'll try an dig that up for you.