Ok, well I only got an airbrush ( a cheap-ass one that only does wide areas) 3 years ago, still don't use it a lot. Only if I have to paint like a car body in gloss that is. And still it's really hard and way messier than brushes.
Got tons of tips, but here's how I do weathering:
depending on the part you're weathering, choose a color. Metal choose some kind of silver. My experience is that Revell metallic silvers are not as appropriate for this, the best paint for this is my Humbrol nr 11 silver. That one is way smoother and spreads better.
For cockpit dials use white, and in some occasions you could try a lighter or darker tint of paint of the color you're weathering, experimenting is key here.
Anyway, gets down to this: have a DRY brush (not dipped in white spirit yet) and get some weathering paint on it. Then start wiping the paint off to a piece of cardboard (not on the same spot all the time) until there's only a little bit of paint coming off in dry swipes. Then, just wipe over your part, the paint should stick to the elevated areas. The key here is not overdoing it, watch out with that. If you do well (not that hard, takes some practice) you'll get nice effects. Like a tank that has chipped paint, or an engine or gun that looks used.
I use this extensively, here's some examples:
http://users.pandora.be/b_corijn/diorama/churchill.htm (huge load btw)
(on the Churchill you can see that a wetter form of drybrushing can be used for mud)
(btw, do you only have ONE brush?)