Sam wanted me to describe how to bounce-spray and dust with canned spray-paint in order to get some airbrush-style effects. Since I worked them out myself, I don't know what other painters call them, but I refer to them as bounce-spraying and dusting.
Bounce-Spraying is not spraying the paint directly on the model, but rather 'bouncing' it off the painting surface up onto the model, (or off of a piece of cardboard held in your hand). It's how I got that nice dirty look on the legs of my Dark Vengeance models without pulling out the airbrush.
Notice the spray pattern on the table. You spray in front of where you want the paint to go. The paint will bounce up in the direction that you're standing, so pay attention to your positioning around the model so that you get even coverage all-around.
You can see the final effect especially well on the Dark Angels above.
When you do the bounce right, the top of your model's base won't have any paint on it at all, while the sides of the base and the legs of your minis will have a nice gradation of color on them. It's enormously wasteful of paint, because you're painting the table surface more than the model, but the results are fast and easy. It also works much better with fine-droplet paints like the Tamaya enamels I use. Standard Rustoleum spray paints have droplets that are much too large for bouncing onto small models.
Here's some of my DV Chaos Cultists. They've had one round of Devlin Mud wash after the bouncing and dusting, but I haven't yet done any dry-brushing or detailing on them, as I did with the Chosen who looked exactly the same at one point.
Dusting is holding the can 2-3 times further back than you normally would and shaking it around as you spray lightly. Rustoleum works fine for this. Your goal isn't coverage, it's to break up the nice clean color of your basecoat and make it look a bit dirtier/blotchier. I used black, orange, red, and a bit of sand colored paint for this. Stay subtle!
That's it in a nutshell. I want to get these models done quick, and this method is working pretty well for me. Enjoy!