Wow, a pretty wide open topic, but here's a few of my thoughts from my vast experience of building 1 (one) 1/72 scale tank:
- get some artist pastels (not the oil kind). Scrape some powder off of them and apply the dust with a brush. Keep building up various earth tones in areas of the model that would collect dirt
- get some photos of weathered armor. Examine wher the wear and dirt is and replicate this type of thing on your model.
- put the decals on before you weather and then weather right over the decals.
- learn how to dry-brush and use this technique with lighter colors on various metal edges, bolts, etc.
- flat paints can be 'buffed' a little bit to simulate some wear. Rub various sections with a q-tip or piece of cotton and you'll see that is will create a bit of a different texture to the paint that looks pretty realistic.
- I've seen lots of good armor that has a 'wash' applied to highlight all of the nooks and crannies of the model. If you're not sure what a 'wash' is, do a search through various newsgroups or FAQs and you'll find tons of techniques/advice.
- You can simulate paint chips with silver paint, but the best stuff I've seen is when the paint is really chipped. I've seen guys that will apply a base coat of metallic/steel color paint, and then seal it with future. Then they apply the finish coat over the model. Then, they either take some tape, sandpaper, or a knife and just start 'pealing' away at the top layer of paint. The metal color shows through and it looks pretty good. I imaging it takes some practice though.
Weathering is a bit of an 'art' and the most successful guys I know will just say that they keep on using different techniques until it 'looks right'. It's not tough, but you have to have an eye for it to make it look really good...
M.