Continuing with the theme of Spifires for weathering examples...
(btw, Woodbeck, that's a nice Spit you have there. The slight angle of the rudder is a nice touch)
The streaking on the wing was done using a very light black wash, lightly pulling the brush back along the wings. I also used some pastel dust, black and some browns, applied with a Q-Tip, again scrubbing back along the wings. Everything was then sealed with a coat of clear.
The main thing with weathering, particularly aircraft, is to go light and work from there. You can always add more weathering, but it is difficult to undo excessive weathering.
Another technique that can come in handy is dry-brushing, whereby you dip an old brush into a bit of paint, wipe most of it off, in then kind of scrub with varing degrees of pressure. For this, you want to use an old brush, perhaps even cutting the bristles short. You can achieve a subtle effect with light pressure or bigger effects with more pressure. For the photo above, I dry brushed silver on the wing root where the pilot's and ground crew's boots have worn away the paint. This is a heavier effect than what I would normally use, but when looking at Spitfire photos, I would often see that this paint would be severely worn, even when the rest of the kite looked pretty good. You can drybrush on you wings an fuselage as well, not only making streaks but even just to kind of break the up the colors just a bit.
Like everything else about this hobby, there is no on "right" way of doing things. Experimant and have fun!
Andy