Matt
Scratch the wood (parallel to the boards) with a wire brush, lightly sand to get rid of the fuzz, then apply an India Ink/Alcohol wash. Brush parallel to the boards Let that dry, and apply another wash in some random boards. Dab on some rust spots (dark rust colored chalk in a few spots of you want.
Others will have their own technique, but I have found this to be cheap, quick, and effective