Some of the weather advice our club has liked.
http://web.mac.com/ksoc/S.C.A.T./Tec...ring_Tips.html
The Boys at DAK (Desert Armor Korps) have created some great guides. If I remember correctly you can do a lot of the weathering without an airbrush.
http://www.desertarmor.com/DAK2/DAK_PAINTING_GUIDE.html