My scene will be the street of a ruined town. What about dust from the buildings? I need some on my armor. Any tips on creating dust?

the method you use to create dust depends on if you want the effect to be permenant or not, if you want a permemant dusty effect then i would recommend using multiple washes of a dust coloured paint focussing on horizontal surfaces ( where dust collects on real afvs ) and corners. if you want a removable effect then pastels would be better. if you want more detail of these methods id be happy to help!

Another option is to airbrush some Tamiya buff. Use a light coat with low psi and lightly cover the lower portions of everything. AFV's, men, buildings.....

You can also go the pastel chalk route. If you've never used them, get the chalk kind not oil. You scrape them into a container with a knife and dip your brush in them and apply to 'affected' areas. You can use a soft brush and use a stroking motion, or a stiff brush and use a dabbing motion. It just depends on which look you want.

You can use a combination of the ways suggested also.

Thanks for the tips. The effect I am looking for is tempoary. For instance, a building just gets blown apart and there is dust everywhere. Debries and dust all over the road and so on. Just a light coating on the tank. I may use the pastels.

Good choice - go pastels. You can even try things like 'siffting' the pastel dust over.
Give multiple colors a try too. Vary the shade a bit. They will mix and blend just fine.

I agree with slodder use pastels and cover them with an matt varnish at the end.

I use colored chalk at times, and use a pipette to direct the location of the dust. I also use the super-fine shavings from balsa foam to create dust from buildings, but it is only available in a mustard-yellow color... so it really depends on the nature of the structure itself.

Another idea for creating debris... make a lot of "bricks" (see article in Digital Diamonds) and then put a quantity inside of a cloth bag and smack them repeatidly with a hammer... then you have perfect bits of "brick fragments" to add to your scene. Throw in some wood strip and other materials for a more realistic "mix" of materials typically found when a building is destroyed... and don't forget to add some wires dangling from buildings, windows, and ceilings!