Sometimes I just grab some rusty-coloured paints and wing it. Humbrol has a lot of useful colours for that. A Dremel can come in handy for drilling holes, or use the rotary wire brush attachment for thinning away the back side of a panel to create torn metal that's rusted through.
Last year I picked up some Deluxe Materials Scenic Rust from a train hobby shop, though I've yet to try it. Looks very impressive though; it relies on a two-step chemical reaction to create rust.
As always, try it on something unimportant first to refine your technique.