Well, the Rustbukkits are an extreme case of weathered vehicle, but I imagine the process can be applied anywhere, and even combined with other techniques.
What I did was a simple two tone basecoat and drybrush for the corroded steel, or wash for the bronze. My trukk and my battlewagon both have different colors of rust though, because I had to make a new mix for the battlewagon. It was just a red, an orange, and a dark brown mixed together with a dark brown/gold metallic paint. I couldn't tell you the equivalents in
GW terms, as I'm using super cheap craft paint I picked up at a craft store. Then I just drybrushed a heavy bit of boltgun metal on top of it moving my brush in the direction that things would be scraping against the surface. After that, I washed it with badab black and BAM. Near instant rust.
The patina mix I used was a little more complicated, I mixed a little too many colors in and couldn't get it right the first go. Started with a turquoise color, a mid tone green, and a light baby blue that I mixed with a ton of water and a little bit of dish soap. I washed it over the top of a bronzey color that I got and wiped up anything that was too thick. In other places I didn't let it flow all the way to give it a look similar to the picture of the inside of the cab. Those two panels inside there were my test pieces and got two washes of slightly different colors and several mop ups with a paper towel until I figured out what I wanted.
I probably should have taken pictures of myself working, I guess. Anyway, easy techniques that I adapted from things I've seen floating around the internet.
http://www.youtube.com/watch?v=i6OSyAT7-Ys
This video helped me figure out how to do the bronze, but I poured on the patina mix a little thicker to make the metal look duller and more worn. For the rust though, I prefer drybrushing gunmetal over rust red.
Edit: An interesting property of plastic glue is that it distorts the plastic it's placed on. Which can be both good and bad, depending on the purpose. You can see it above the back door that goes to the bed of the battlewagon. There, it looks fine, like a really shoddy welding job, or just even more corrosion. So it may be worth playing around with. I hate my tube of Testor's plastic cement. It just constantly pours out way too much at a time and is unstoppable. I often apply it with a toothpick or just use Cyanoacrylate.
Here are some shots of my infantry that I've either fully painted or started converting. I got the power klaw idea from a post on da WAAAGH, and used the slugga boy as a test model for painting brown skin instead of green. That's why he has both colors on his arms, to compare them side by side. He obviously had some bad serjury.