Definitely downward streaks - as per the prototype thanks to Mother Nature.
Citadel (or other brand) inks or "washes" are one effective way, but any regular paint can also be thinned to make your own wash.
"Dry brushing" (dip the brush in the paint, then remove most of it before lightly passing the brush over the model) is a way of picking out detail (like rivets, wood grain etc), and also has its place during weathering.
There is an excellent video on You Tube that was part of a garden railway series -
http://www.youtube.com/watch?v=dhNhhYR2x-M
That's the link to part one, part two will be easy to find from there
While you're on You Tube, just check out "Weathering Techniques - (and don't ignore the military modellers), and discover the wonders of masking, hair spray etc, etc