Hey J-man
I posted this quick weathering guide in the Techniques Forum not too long ago. It gives a brief rundown on some of the methods used. There are other posts that touch on this subject in the Armour Forum & the Techniques Forum.
Hope this helps.
Washes - using a small amount of paint (usually a shade or two darker than the base coat) mixed with thinner. This is applied to the model and is allowed to flow into the cracks and crevices giving depth to the kit.
Drybrushing - load your paintbrush with paint then remove most of the paint by scrubbing it out on a piece of cardboard. Then using a scrubbing action, apply small amounts of the paint to the model. This method highlights rivets, bolts, etc.
The colour that I usually use for this is steel because I'm trying to show that the paint has worn off and is exposing the metal beneath.
Pastels - these are artists chalks (not wax) that are applied to the model to create dirty and dusty areas as well as toning down the original paint colour. I sand these down to a fine dust and apply them with a ratty old paint brush.
I also will use ground pencil leads to metalize the look of some points on my models.
Tracks - for metal tracks I use a base coat of gun metal acrylic followed by a heavy drybrushing of rust. Then I will drybrush steel to highlight the tracks and finally apply ground pencil lead using my finger to burnish the tracks.
Each and everyone of these techniques and others that will be mentioned in this thread require a certain amount of practice. Try them out on an old kit or a piece of plastic sprue until you feel confident with them. I have seen far too many models that were built great but then ruined by excessive and poor weathering. Practice, practice, practice and remember a little goes a long way.
Happy Modelling and God Bless
Robert